Chapter 7: Phylum Echinodermata


7.1 Introduction

Echinoderms include common seashore animals such as seastars (also known as "starfish"), sand dollars and sea urchins, along with hundreds of more exotic forms. Their basic body plan is very different from other animals, but their closest living relatives are the Phylum Chordata (which includes the vertebrates).

Echinoderms are exclusively marine, and most are benthic. They are present in virtually all marine environments of normal salinity, from the shallow intertidal to the abyssal zone. Many echinoderms are suspension feeders, while others are predators, scavengers and herbivores. A few are deposit feeders.

Although the phylum is quite diverse, echinoderm physiology and their body plan display a surprising uniformity. They are characterized by an internal skeleton (endoskeleton) composed of calcitic plates (ossicles), and a water vascular system. The ossicles have a porous microstructure that is distinctive. A major feature of the skeleton is that the ossicles may increase in size during the growth of the animal. The main portion of the body skeleton, known as the theca or calyx in most echinoderms, may have accessory appendages (arms, rays, stem or brachioles).

The water vascular system is an interesting system unknown in any other phylum. In ancient echinoderms, water circulated through pores in the body wall and was apparently important for respiration and feeding. More derived taxa have a specialized system where the water is drawn through a sieve plate (madreporite) by the action of cilia or internal pumping. The water enters a calcified tube and is directed to various parts of the animal. The water eventually fills small sacs inside external tube-like extensions (the tube feet or podia) along the rays and these, through hydraulic manipulation, may pulsate to move the animal through the environment or transport food to the mouth.

Echinoderms are generally radially symmetric, with adults displaying a secondary pentaradial symmetry. The symmetry is secondary, because echinoderm larvae are bilaterally symmetric. One group, the sea cucumbers, developed a tertiary bilateral symmetry. The mouth is located centrally on the upper or lower surface of the animal (oral surface), or at the anterior extremity. The other surface is termed the aboral surface. A coiled gut extends from the mouth to an anus, which is situated between two rays or at the posterior end. Echinoderms have a well-developed nervous system and reproductive system, but no heart (no need with the water vascular system).

7.2 Classification

Phylum Echinodermata contains over a dozen classes, about half of which are known only from the Paleozoic. They are classified by characters such as the general morphology, ossicle structure, arrangement of the water vascular system, and embryology.

7.2.1 Subphylum Homalozoa*

The homalozoans include the "carpoid" echinoderms and possibly another minor group. Carpoids are small and rare fossils found only in Lower and Middle Paleozoic rocks. They have an asymmetric, flattened body composed of calcitic plates, and a short stem called an aulacophore. Carpoids have been assigned to the Echinodermata because the calcite of their plates has a characteristic echinoderm microstructure, and because most bear a food groove of some type.

Figure 7.1  Dendrocystites, a carpoid.  Note the absence of radial symmetry (Brusca, 1990)

7.2.2 Subphylum Pelmatozoa*

Pelmatzoans are ehinoderms that are radially symmetrical to some degree, have a generally cupshaped body (theca) enclosing the viscera, and possess food-gathering appendages (arms or brachioles) extending from the theca. Most pelmatozoans have a jointed stem that is usually used to attach the animal to the substrate.

Class Crinoidea*

Figure 7.2  Botryocrinus, a stalked, fossil crinoid (Brusca, 1990).


Class Blastozoa*

Figure 7.3  A generalized cystoid (Brusca, 1990).



Class Blastoidea*

Figure 7.4   Morphology of a blastoid.  (A) Side view of the calyx of the Mississippian blastoid Pentremites (Prothero, 1998); (B) Top view of the same blastoid (Prothero, 1998); (C) a generalized blastoid from the Carboniferous (Brusca, 1990).


7.2.3 Subphylum Eleutherozoa*

Class Asteroidea*


Figure 7.5  Left:seastar Luidia phragma  (Brusca, 1990); Right: Aboral view of Ctenodiscus (Asteroidea).  The ambulacral radii are labeled according to convention (Brusca, 1990)


Class Ophiuroidea*

Many ophiuroids are deposit feeders, while some capture suspended food or small prey with their podia (suspension feeding). The mouth is centrally located on the lower side and leads to a blind gut with no anus.


Figure 7.6  Left: A brittle star, Ophiopholis aculeate (Brusca, 1990); Right: The ophiuroid Asteronyx crawling on a gorgonian.  Note the bighly articulate arms (Brusca, 1990).




Class Edriasteroidea*

Class Echinoidea*

Regular echinoids can be distinguished easily from irregular echinoids by their circular test, nearly perfect pentameral symmetry, and the central location of the anus (directly above the mouth). The ambulacra have 2 or more columns of plates. The interambulacra have one or more columns of plates and are all similar. The spines are generally long and an Aristotle's lantern occurs in all taxa. All Paleozoic echinoids were regular.

Figure 7.7  Anatomy of the regular echinoids.  Left: Lateral view of an echinoid test; Top Right: oral view; Bottom Right: aboral view: (Prothero, 1998).




Irregular echinoids are distinctively elongate in the adult stage. This shape difference as well as the posterior position of the anus (instead of dorsally, like the regular echinoids) are the two most telltale differences setting the two types apart. Irregulars also usually have petals on the upper surface, and each ambulacrum and interambulacrum has 2 columns of plates (with the exception of the posterior ambulacrum, which differs from the others). Spines are generally short and Aristotle's lantern is absent in most adult forms, except for the sand dollars. The irregulars underwent a spectacular radiation in the Mesozoic and are much more common as fossils, compared to the regulars. The derived irregulars also have concentrated the respiratory devices on the aboral surface, and have developed food grooves.

Figure 7.8  Irregular echinoids.  Specimens are 7-15 cm in diameter. (A) Clypeaster (Eocene-Recent), dorsal view. (B) Mellita (Miocene-Recent), dorsal view. (C,D) Micraster (Cretaceous-Paleocene), dorsal and ventral views (Stearn,1989).


Class Holothuroidea*

These are the sea cucumbers, which do not superficially resemble any of the other echinoderms. Close examination however reveals that they do have a pentaradial symmetry, but the anus is opposite the mouth on an elongated oral-aboral axis. The calcitic plates are reduced to dermal, microscopic sclerites, which are often used in classification schemes. They have a water vascular system and podia. Holothurians are generally deposit feeders- they use small tentacles surrounding the mouth for particle collection. Several species are suspension feeders. A few rare forms are planktonic.


Figure 7.9  Left: Parastichopus in its feeding posture.  Right: Release of Cuvierian tubules (defensive structures) by Holothuria (after Barnes 1980; from Brusca, 1990).






7.3 Terminology

pentameral symmetry
water vascular
central disk
ambulacral groove

bilateral symmetry
colomic cavity
regular echinoid
Aristotle's lantern

blind gut











7.4  Questions

1. Asteroids are commonly found in marine shallow water. Locate the following features: oral and aboral surfaces, mouth, ambulacral grooves, podia, madreporite. Sketch. (See Appendix, Figure 7.10 for help)




2. Examining a recent brittle star, locate the following: central disc, oral and aboral surfaces, mouth. Sketch. (See Appendix, Figure 7.11 for help)





3. Looking at the recently expired sea urchins, you should be able to find: oral and aboral surfaces, spines, mouth, anus, madreporite, ambulacra and interambulacra. Sketch. (See Appendix, Figures 7.12 through 7.14 for help).





4. Looking at the pieces of urchin test, what is the function of the small paired holes? What are the bumps on the surface? (Figures 7.13 and 7.14 may give you a hint)…




5. Locate "anterior", "posterior", ambulacral and interambulacral areas, axis of bilateral symmetry. Describe the relict pentameral symmetry. Sketch.







6. Looking at the broken sand dollar test, what function do you think the internal struts perform?




7. Examine these fossil crinoid calyxes and stems. The calyx is generally not fossilized, because the plates composing it usually disarticulate soon after the animal dies. Sketch a typical crinoid, labeling calyx, stalk, arms and pinnules.







Echinoderm taxonomic characters


1.  At the species level:

a.  Each group has a bag containing two specimens of Recent sand dollars --  irregular echinoids – from the northern Gulf of California.  One specimen belongs to the species Encope grandis, the other specimen belongs to the species Encope micropora.  Ask your T.A. to tell you which species is which.

            What morphological features distinguish these two species?  Describe at least three morphological differences.






b.  A third bag contains a fossil specimen from the Pleistocene of the northern Gulf of California.  Which of the two Recent species does this fossil most closely resemble?  Justify your answer by describing the similarities.













2.       At the class level:

You’ve now seen representatives of all the major classes of echinoderms.  Pick three of the classes and describe the key morphological differences that distinguish one class from another.  Use the display specimens to help you answer this question and refer to one or more display specimens in your answer.





Figure 7.10  Morphology of the asterozoans; (a) ventral surface; (b) dorsal surface (Benton & Harper, 1997)









Figure 7.11  Morphology of a living ophiuroid showing the central disk and long thin arms with vertebrae and enclosing plates, as well as tube feet in ambulacral groove (Boardman 1987)

















Figure 7.12  Internal anatomy of echinoids.  (A) A regular sea urchin (vertical section). (B) internal anatomy of Arbacia  (Brusca, 1990)


Figure 7.13   Cutaway view showing the internal anatomy of an echinoid (Boardman et al,1987)








Figure 7.14  External anatomy of an echinoid, Echinus; Left: Top surface, Right: Bottom surface.  (Top surface by Durham, J.W., and bottom after MacBride, E.W., et al., In: Moore, R.C. , editor. Treatise on invertebrate paleontology, Part U, Echinodermata 3. New York and Lawrence, KS: Geological Society of America and University of Kansas Press; 1966; from Boardman et al, 1987).

Chapter 8: Superphylum Arthropoda

8.1 Introduction

Approximately three-fourths of all known animals are arthropods-they constitute the largest phylum in the animal kingdom (estimated at well over one million extant species). Forms familiar to us are the insects, spiders, crabs and trilobites. Many of the first complex fossilized animals were arthropods and, if their rapid adaptations to changing environments are any indication, arthropods will probably be the last living animals on Earth. They have exploited every known environment; arthropods are found at all depths in marine, brackish, and fresh waters. They are common in hot springs, saline lakes, ephemeral ponds, and underground rivers. On dry land they are found in deserts, forests, grasslands and tundra. Many of the species are adapted to parasitism. Indeed, of all animals with the possible exception of humans, the arthropods have made the greatest impact on the Earth's biosphere.

Figure 8.1  Some of the main arthropod groups: a variety of forms based on a simple body plan of a tough exoskeleton and jointed limbs (from Benton, 1997).

Arthropods have segmented bodies with two or more distinct regions termed tagma. The body is covered by a tough, sometimes flexible, chitinous exoskeleton that serves for both protection and the attachment of muscles and other soft tissues. This covering is a simple device that may explain in large part the startling success of the group. It can be mineralized selectively to enhance its protective function and yet maintain sufficient articulating ability. It is the basis for a variety of arthropod innovations, from claws and antennae to wings. In terrestrial arthropods, it serves as an excellent barrier to dehydration.

The external nature of the exoskeleton does have one limitation-it does not grow continuously with the internal soft parts of the animal. To increase in size, many arthropods must periodically shed the exoskeleton and secrete a larger one, a process known as molting or ecdysis. The discarded molts, or exuviae are often preserved as fossils.

Each tagma of an arthropod, including those fused together, typically bears a pair of jointed appendages. In the head region, one or two pairs are modified into long sensory structures called antennae or antennules. Tooth-like jaw appendages termed mandibles, are also usually present. Maxillae are limbs in the head region that are modified to pass food to the mouth. Most of the other appendages function as walking or swimming limbs; in aquatic forms they may have attached gills. In one group (the chelicerates), the first pair of appendages have pincer-like arrangements at their tips termed chelicerae, and there are no antennae.

Arthropod classification and phylogeny have always been controversial. It is obvious, for many reasons that we cannot go into here, that the arthropods are all descended from polychaete (Phylum Annelida; segmented, chitinous marine worms) ancestors. The controversy centers around whether all the arthropod groups share a single annelid ancestor, or if there have been several instances of annelid worms giving rise to an "arthropod" group (most likely by the development of tagma from the segmented condition). For years, conventional wisdom has held that the Arthropoda constitute a polyphyletic group, and each major group of arthropods therefore represents a separate phylum. More recent phylogenetic work however, both molecular and morphological (based primarily on Cambrian and other fossil taxa), seems to support arthropod monophyly. In this lab we will therefore list Arthropoda as a superphylum, and the individual groups as separate phyla. The phyla are Trilobitomorpha, Crustacea, Chelicerata and Uniramia. There are also a number of minor and obviously related phyla that we will not be able to cover. Incidentally, monophyly of the Arthropoda makes the evolution of the arthropods the single largest adaptive radiation in the history of eukaryotic life.


8.2 Phylum Trilobita

Figure 8.2  Flexicalymene meeki, Waynesville Formation, Upper Ordovician (from Levi-Setti, 1993)


Trilobites are exclusively Paleozoic fossils found throughout the world. Though among the earliest of the fossilized invertebrates, trilobites are in many ways the most advanced of the already complex arthropods. Because of their extreme diversity of form, and their abundance in certain rocks, trilobites are always popular fossils for study.

The term "trilobite" refers to the division of the anterior tagma into three longitudinal parts: an axial lobe in the center, and two flanking pleural lobes. Trilobites are also divided transversely into three tagma: the head (cephalon), a middle section called the thorax, and a "tail", properly termed the pygidium. Like most arthropods, these parts are covered with a hard exoskeleton, which in trilobites is made of CaCO3, chitin, and sometimes CaPO4.

The cephalon of trilobites bears a variety of structures important for both the classification of the phylum and for the interpretation of their life modes. The axial portion of the cepahlon is inflated to varying degrees and termed the glabella. Most members of the phylum have two eyes on either side of the glabella, each of which usually contains several lenses. All trilobites, except the agnostids, have facial sutures on the cephalon that mark where the exoskeleton would split during ecdysis. These sutures are faint lines that follow three patterns; protoparian and hypoparian sutures are confined to the cephalic margin. Proparian sutures are mostly dorsal but do not meet the posterior dorsal margin of the cephalon. Opisthoparian sutures are predominantly dorsal and do meet the posterior dorsal cephalic margin. The portions of the cephalon on both sides of the glabella and inside the boundaries of the proparian and opisthoparian facial sutures are called fixed cheeks. The two dorsal cephalic parts outside the sutures are termed free cheeks. The posterior lateral corners of the cephalon sometimes bear a set of long extensions called genal spines. The hypostome is a plate found on the underside of the cephalon in front of the mouth.

The trilobite thorax consists of at least two (usually many more) unfused, articulated segments. Those portions that are part of the pleural lobes are called pleura (singular pleuron). Pleural spines are pointed lateral extensions of the pleura.

The pygidium of trilobites is made of anywhere from one to thirty fused segments. It may also bear spines along the margin.

Trilobite appendages have only been found rarely-their exoskeleton was not mineralized like the rest of the body. The limbs of some trilobites were biramous, meaning that there were two appendages (exopodite and endopodite) extending from the basal unit (protopodite). Very often one of the extensions is a gill. A few trilobites have been found with attached antennae.

Trilobite eyes are especially interesting because they are the most ancient fossilized visual systems (though, contrary to popular claims by some paleontologists, they most likely do not represent the oldest visual systems, nor do they tell us much about the evolution of animal sight). Great variation in morphology, structure and size of trilobite eyes suggests differential use of vision among different types. The eyes are usually crescentic in shape, but can be globose, conical, stalked or fused anteriorly into a single band. The composition of the eye lenses account for their unusual preservation. Each lens is composed of a single calcite crystal oriented with its principal optic axis (the c axis) normal to the visual surface. Such precise crystal orientation is functionally desirable because it eliminates polarized rays and does not produce a double image. There are two basic types of trilobite eyes: holochroal and schizochroal. Holochroal eyes are characterized by close packing of biconvex lenses beneath a single cornea. These lenses are generally hexagonal in outline and range in number from one to more than 15,000! Schizochroal eyes on the other hand are an aggregated type of eye. These eyes are made up of a few to more than 700 relatively large, thick lenses, each covered by a separate cornea. Each lens is positioned in a cylindrical mounting and is separated from its neighbours by cuticular-type material.


Figure 8.3  Triobite eyes. (A) Holochroal eye composed of closely packed lenses (Levi-Setti, 1993) (B) Schizochoroal eye with lenses separated by solid cuticle.  (from Prothero, 1998).

8.2.1 Classification

Many of the major groups of trilobites appeared rather suddenly in the early Cambrian with derived characters and non-obvious interrelationships. A previous scheme of classification used the type of facial sutures as a primary taxonomic character, but subsequent work showed that these features were often very derived. We will use the system adopted in the Treatise of Invertebrate Paleontology;

It has gradually become evident through ontogenetic studies that phylogenetic classification of the trilobites must be based on many characters and great care is necessary in their selection. Part of the phylum has been analyzed cladistically and it will probably be one of the first invertebrate phyla to be completely analyzed cladistically (along with the brachiopods and perhaps echinoderms). This is because the arthropods have more discrete morphological characters in general compared to such groups as the molluscs.

Some of the most exceptional specimens of Cambrian trilobites come from the Burgess Shale in British Columbia, Canada. It is from these Konservat-Lagerstatten forms that we have gotten much of our understanding of trilobite soft anatomy. This aids in functional morphological analysis, as well as systematic study of these extinct critters. The Burgess Shale itself is interpreted as being deposited, in part, as a slumped block of fine-grained material. The slumping is an important aspect of the preservation, for it placed the entire block into anaerobic water, not only killing the organisms, but keeping them from serious decay and postmortem scavenging. Rapid burial of the block entombed them and the fine-grained dark grey-black fossil-rich rocks are the result.

8.2.2 Taxonomy

Order Agnostida

Order Redlichiida

Order Ptychopariida

Order Phacopida

8.2.3 Terminology

gill branch

axial furrow

free cheeks
genal spine
walking leg





8.3 Phylum Crustacea

With the possible exception of the trilobites, the crustaceans are the most common arthropods in the fossil record. They are almost entirely aquatic, being found in fresh, brackish and marine waters (terrestrial isopods and semi-terrestrial crabs are the only exceptions). Crustaceans have two pairs of antennae, and one pair of antennules. All appendages, except the antennules, are biramous, or secondarily uniramous. Three classes are important as fossils. 12.3.1 Classification

Class Ostracoda


Figure 8.4  Top: generalized ostracode with left valve removed to show appendages. Bottom Left: exterior view of right valve of Microcytherura (Pleistocene x 70).  Bottom Right: exterior view of left valve of Orionina (Pliocene, x 58)  (from Boardman et al 1987).


Class Cirripedia

Figure 8.5  Sessile (acorn) barnacles, Semibalanus balanoides (from Brusca, 1990).


Class Malacostraca

Figure 8.6  The diversity of malacostracan crustaceans is illustrated by these representative taxa: (A) The phyllocarid Ceratiocaris (Silurian); (B) The syncarid Palaeocaris (Carboniferous); (C) The eocarid Tealliocaris (Carboniferous); (D) The hoplocarid Acanthosquilla (Recent); (E) The decapod Eryma (Jurassic)  (Clarkson, 1993; from Prothero, 1990).


8.4 Phylum Chelicerata

The chelicerates (sometimes called the Cheliceriformes) are one of the most successful animal phyla, including such familiar animals as spiders, scorpions, ticks and mites. They are also one of the oldest, being well established in the Cambrian. They are distinguished generally by having the first two tagma fused, and in general eight limbs. The first one or two pairs of limbs are often modified for feeding.

8.4.1 Classification

Class Merostomata

The typical merostome body is divided into two parts; a cephalothorax (or prosoma), and an abdominal tagma, the opisthosoma. They have anterior claws (chelicerae), and a spike like extension at the posterior end (telson). The opisthosomal appendages are biramous, with one branch serving as a gill. Merostomes are divided into two subclasses:

Figure 8.7  Morphology of the recent horse shoe crab Limulus; Left: dorsal side , Right: ventral side (from Boardman et al, 1987).


Figure 8.8  Morphology of Eurypterus from the Silurian; Left: dorsal side; Right: ventral side (from Boardman et al, 1987).





Class Arachnida

Arachnids include the scorpions, spiders, ticks, mites and other related forms. The typical arachnid body is divided into a cephalothorax and an abdomen (except in ticks and mites, where they are fused together). The cephalothorax bears six pairs of uniramous appendages (only one extension from the basal protopodite), the first two of which are modified for feeding, and the last four for locomotion. Like insects, all arachnids are air-breathing. The earliest known arachnid is a well preserved scorpion from the Silurian of Sweden. The scorpions were at this time an exclusively freshwater group!

8.5 Phylum Uniramia

The Uniramia, so named for their exclusively uniramous appendages, include the millipedes, centipedes and insects. This phylum represents the single most successful evolutionary event in the history of multicellular life.

8.5.1 Classification

Class Myriapoda

Figure 8.9  A millipede from East Africa (from Brusca, 1990).

Figure 8.10  A California centipede (from Brusca, 1990).


Class Insecta

The insects are by far the most diverse and common arthropods today. The typical insect body is divided into three distinct tagma: the head (composed of six segments), thorax (three segments), and the abdomen (eleven segments, usually with only ten apparent). The head bears a pair compound eyes, antennae, mandibles, and maxillae. Each of the three segments of the thorax have uniramous (unbranched) legs and, in most insects, the posterior two thoracic segments bear one or two pairs of wings.

Insects appear to have developed from an arthropod group that was already breathing air. They are mostly terrestrial-the few aquatic insects are usually able to live in the water by trapping air bubbles against their respiratory openings (spiracles). Terrestrial insects are one of only two groups of animals that are truly adapted to terrestrial life, in that they manage water to an extent that allows them to explore the terrestrial habitat fully (the other group is the Amniota, which includes Mammalia and Reptilia).

Wingless insects are known from rocks as old as Devonian, and the winged forms appear first in the Pennsylvanian. Most Paleozoic insects are known from swamp deposits, and most Mesozoic and Cenozoic fossils from air-fall tuffs and amber (although there are several significant Lagerstaten).

Figure 8.11  Diversification of the insects (from Prothero, 1990).

8.6 Terminology





8.7 Questions


Phylum Trilobita

1. Look at the numerous specimens. Note that some of these specimens seem to have been molts. How can you tell? Note also that the specimens vary considerably in size. What accounts for this variation? Sketch an individual and label the following: glabella, cephalon, thoracic segment, pygidium.



  2. Flexicalyrnene is one of the earliest genera to have the capability to enroll. Some specimens show this nicely. What advantage can you think of which might have led to selection for enrollment? Phacops is another trilobite from the Devonian which enrolled.




3. These samples show how densely fossils can accumulate in sediment. Does this necessarily reflect the real density of the living population. Why or why not? (Think of energy regimes, and also note the disarticulation and fragmentation of these fossils).




Other Arthropods

4. Barnacles are among the most abundant modern crustacean groups. They also have a geologic history (Ord.), but they are rarely seen in rocks older than the Tertiary. Look at these fossil specimens. What physiological and environmental factors may control their preservation?




5. Examine the examples of a modern horseshoe crab and look at the fossil representatives.  How have they changed in appearance?




 6. Look at the examples of fossil insects on display. Note how diversified their morphologies are. Give a scenario for how these insects were preserved.



Comparative anatomy of arthropods


You will be given handouts that contain schematic drawings of representatives of major groups of  arthropods:  trilobites (a trilobite), insects (cockroach), crustaceans (crayfish), chelicerates (a scorpion and a eurypterid).


Arthropod anatomy can be said to be variations on the themes of segmentation and specialization; some segments are repeated while others are fused; some limbs remain generalized while others perform specialized functions.


1.  Label the following morphological features on the schematic drawings ( Please note: some features may not be present in all  groups):



Cephalothorax (or Prosoma)




Walking limbs

Feeding limbs





2.      For each group illustrated, is it:

  1. marine or terrestrial?  What’s it the evidence?
  2. If marine, benthic or pelagic?  What is the evidence?
  3. If terrestrial, walking or flying?  What is the evidence?



3.  Arthropods are often said to be the most “successful” group of animals.  But that, of course, depends on one’s criterion of “evolutionary success”.  Briefly discuss two criteria that could be used to measure evolutionary success.  How do arthropods measure up by each criterion?