ECHINODEEMATA 221 



mon than in earlier geological times, when they must have been very abundant 

 and very beautiful. They contribute much to the formation of the limestone of 

 the Palaeozoic. They are usually provided with jointed stalks, by which they may 

 be attached to the bottom. At the summit of the stalk is a central disc with five 

 arms often much branched and bearing lateral pinnules. The anus is on the oral 

 or upper surface, the stalk arising from aboral surface. They are inhabitants of 

 both shallow water and deep seas. 



Class IV. Asteroidea (starfishes) (Fig. 97). — The asteroids, of which there 

 are several hundred species, are free echinoderms with a central disc and usually 

 five arms. The latter are large and contain liberal ccdomic spaces in which are 

 lodged outgrowths of the digestive system and other organs. There is a distinct 

 oral and aboral surface. The anus and madreporic body are on the latter. Dis- 

 tinct ambulacral grooves lie on the oral surface of the arms. Adult starfish may 

 vary in size from a few inches to two feet or more in diameter. 



Class V. Ophiuroidea {serpent-stars). — These are fragile, free echinoderms 

 in which the arms are small and much more distinct from the disc than in the 

 asteroids. The organs of the disc are not all continued into the arms. There is 

 no anus, no ambulacral grooves, and the madreporic body is on the oral surface. 

 Their slender arms are useful in clinging to supports or to prey, and are used in 

 locomotion. They are readUy broken and regenerated. 



Class VI. Echinoidea {sea-urchins, sand-dollars). — These are free echinoderms 

 without free arms. The arms are connected by the development of interradial 

 plates. The calcareous rods are united into plates which produce a complete exter- 

 nal skeleton varying from flat dome-shape (as in sand-dollars) to a globular form 

 (Echinus or Arbacia). The mouth is usually in the centre of the oral surface and 

 the anus near the centre of the aboral, yet one or both may come to have an excen- 

 tric position. In this way the bilateral symmetry is accentuated at the expense of 

 the underlying radial symmetry. The madreporic body is aboral and there are 

 no ambulacral grooves. The spines of the urchins are usually well developed 

 and may be used to scour out rounded pockets in rock in which the animals are 

 sometimes found. 



Class VII. Holothiiroidea (sea-cucumbers). — These are soft, free echinoderms, 

 elongated, cylindrical or flat, with mouth and anus at opposite poles of the hori- 

 zontal long axis. The skeleton is not well-developed, usually being represented 

 merely by scattered spicules. The water-vascular system in most forms com- 

 municates with the body cavity instead of the exterior. Well-developed tentacles 

 occur about the mouth. Most holothurians burrow in the sand or iHud, but others 

 cling to rocks near the surface of the water, and still others occur at great depths 

 in the ocean. Their reactions are rather more complex than in the other members 

 of the phylum. They are more muscular and more responsive to light and con- 

 tacts. In addition to hiding by burrowing, they often contract the muscular wall 

 violently and eject large portions of their viscera. These may be replaced by 

 regeneration. 



257. Suggestive Studies for the Library or Laboratory. 



1. Read and report on the metamorphosis of the various 

 members of the group. 



2. Study from dry and moist material and report on the 



