UNIVERSITY OF THE WEST INDIESDepartment of Life Sciences

BIOL 1263 -- LIVING ORGANISMS II

Lecture summaryECHINODERMATA

The echinoderms are a very distinctive group, with no clear evolutionary relationship to others except distantly to the chordates. There are about 7000 known species, all marine, and mainly bottom-dwelling. They can be very abundant on the ocean floor. Five classes (approximate numbers of known species in brackets) merit mention:

Asteroidea, starfishes or sea stars (2000)Ophiuroidea, brittle stars (2000)Echinoidea, sea-urchins and sand-dollars (1100)Holothuroidea, sea-cucumbers (1100)Crinoidea, sea-lilies and feather stars (800).

The Asteroidea and Ophiuroidea are sometimes treated as subclasses of a combined class Stelleroidea.

In treating body structure, the focus is on three unifying themes: a) a pentamerous body plan, b) an internal skeleton, and c) the water-vascular system.

[In reading the following points, it is suggested to have an illustrated textbook on hand. Illustrations of an ordinary starfish will serve for the features of echinoderms as a whole.]

Pentamerous body planAlthough echinoderm larvae are bilaterally symmetrical, adults

show radial symmetry with structures in fives. This pentamerous plan is unique to the phylum. The origin of pentamery remains a mystery. Echinoderms have a good fossil record, but even the earliest fossil members are pentamerous.

Internal skeletonEchinoderms can extract CaCO2 from sea water, with which they

build a system of ossicles, which may be developed into a strong internal skeleton. Each skeletal element is a single crystal, but usually not solid. Rather, it forms a matrix, which is more economical of material and allows for the insertion of connective tissue. The elements may articulate with each other (e.g. starfishes and brittle stars) or be fused to form a unitary shell (.e.g sea-urchins).

Water-vascular systemThis is a closed system of water-filled tubes and reservoirs. Water

is drawn in through the madreporite (sieve plate), from which it goes

into the stone canal, then to the ring vessel (a circular tube around the mouth) and to the radial vessels. These latter have paired side-branches, each leading to a tube-foot (podium). The main function of the water-vascular system is to supply fluid for these hydraulically operated tube-feet. Note that the system is "closed" in relation to the rest of the body, but not with respect to the surrounding medium. Water is drawn in through the madreporite and can be lost through the tube-feet under pressure.

Tube-feet are tentacle-like structures, which protrude through skin. Water is forced into a tube-foot by contraction elsewhere in the system. Many tube-feet have muscular, bulb-like ampullae. Contraction of the ampulla causes the tube-foot to extend.

Tube-feet are found in all classes of echinoderms, with considerable variation in structure and function, often within a single animal. Their original function is thought to be in respiration, but they can also have major roles in locomotion and sensation. Locomotory functions have apparently evolved independently in three lineages, the Asteroidea, Echinoidea and Holothuroidea.

The nervous system of echinoderms is simple, typically comprising radial nerve cords, with a nerve ring around the mouth. There is no distinct brain.

Two kinds of external structures should be noted. First, in some classes the body surface typically has numerous spines, which give the phylum its name. In some these are long and thin, as in sea-urchins, while of starfishes are typically short and relatively blunt.

Second, a relatively sedentary animal with a large surface area on the sea bottom may be one of very few solid things around, so that it risks being treated as a good settling place for various kinds of larvae. Such an animal must have ways of keeping itself un-encrusted and also needs to keep the body surface clean of debris. Sea-cucumbers do this by means of muscle ripples along the body. The two classes with relatively inflexible exteriors (starfishes and sea-urchins) instead have peculiar little pincer-like spines scattered over upper part of body. These pedicellariae snap at and dislodge any particle or larva that lands on the body surface.

We will now review some points about four classes of echinoderms.

AsteroideaThese tend to be fairly abundant in shallow tropical waters. The

body has a central disk, with mouth in middle the middle on the lower (oral) surface and the anus in middle on the upper (aboral) surface. Project to the sides from this central disk are (usually) five arms, each with an ambulacral groove running along it below. The main of part of

the gut is in the disk, with pouches extending into the arms. The tube-feet extend from the ambulacral groove and mainly function in locomotion.

Primitively, starfishes feed by sweeping particulate matter along the underside of the arms to the mouth by means of the tube-feet. However, some species have evolved to prey on bivalve mollusks. They exert a sustained pull on the bivalve's shell by means of body muscles, with the tube-feet holding fast by means of suckers. The starfish overcomes the bivalve's adductor muscles through endurance and/or brute strength. Only a very narrow gape in shell is needed for the starfish to evert its stomach through its mouth and into the mantle cavity, where the stomach secretes enzymes to digest the bivalve inside its own shell.

Starfishes have considerable ability to regenerate lost parts. This is used by some species for asexual reproduction.

OphiuroideaThese can be hugely abundant on the deep-sea floor. They are

mainly carnivore-scavengers, feeding on live and dead small organisms. Unlike in starfishes, the central disk is distinctly set off from the long, thin, flexible arms. These arms look like snakes and are easily broken. These features account for the names Ophiuroidea and brittle star, respectively.

The ambulacral grooves are closed. This distinctive feature of the class is associated with the use of whole arms as locomotory organs. The gut is entirely contained within the disk and does not extend into the arms, which are mostly made up of ossicles and muscle. The arms have a long reach and can serve as feelers. Associated with this, the tube-feet are mainly sensory (tactile) and do not serve in locomotion.

EchinoideaIt is a useful exercise to imagine transforming a starfish into a sea-

urchin at the level of overall external body organization. This is accomplished in three main steps:

1. Close the ambulacral grooves.2. Curl the arms up, so that their tips touch above and the aboral

surface is restricted to almost nothing.3. Fuse the skeletal plates within each arm, and fuse adjacent

arms, so that the skeleton becomes as a rigid shell.This simple exercise illustrates the overall unity of body plan between two apparently very different animals.

The rigid, usually globose shell, known as the test, is based on interlocking plates that radiate in 20 columns from top down to mouth. Spines are usually very prominent. They are movable and serve mainly defensive and locomotory functions. The tube-feet are also locomotory.

Sea-urchins have varied feeding habits, mainly on living plants, especially encrusting algae. These are removed from the substrate by means of Aristotle's lantern, a complex chewing apparatus in the mouth, comprising 40 skeletal pieces connected by muscles and ligaments, with 5 constantly growing teeth.

HolothuroideaThe body of a sea-cucumber is elongated along the oral-aboral axis

and lies on its side. As a result, there is a measure of bilaterality imposed upon the basic pentamerous plan, although with no very strong differences between the dorsal and ventral sides. Unlike in other echinoderms, it makes sense to speak of anterior and posterior ends to the animal. The body is therefore distinctly worm-like, with the surface bare of spines or any other prominent protrustions.

As in annelids, the body musculature comprises two layers: with the circular muscles on the outside and the longitudinal muscles inside in five broad bands. The skeleton is inconspicuous, with no large outer plates, but many microscopic spicules.

The tube-feet are locomotory in a manner similar to those of sea-urchins. They can reach forward to get a grip on the substrate and then pull the body forward.

Sea-cucumbers have two main feeding habits. In some, five pairs of highly modified tube-feet around the mouth (the tentacles or buccal tube-feet) sweep the substrate for particles and then are retracted to mouth. In some species with this habit, the animal even lives in a burrow and sweeps the surrounding surface with its branches, sticky tentacles, like some tube-dwelling polychaetes.

Some other sea-cucumbers ingest large quantities of substrate, from which they extract nutrients in the gut, like earth worms.

An outstanding peculiarity of this group is found at the hind end of the gut. Branched ingrowths of the cloaca, known as respiratory trees, occupy much of the body cavity. Water is drawn into them through the anus and pumped back out, so that they function as gills.

In some species, when the animal is strongly disturbed it splits the intestinal wall and self-eviscerates through the mouth or anus, spewing out the gut and respiratory trees, which regenerate later. Many have a more sophisticated means to the same end. Very long defensive organs, the cuvierian organs, branch from bases of respiratory trees. The cloacal wall splits at weak points, and the cuvierian organs are forced out through anus. These are sticky and serve to entangle the intruder. Either way, molesting a sea-cucumber can be a very messy business.