PHYLUM ECHINODERMATA 399 



[olothuroidea and Ophiuroidea all the organs of the larva are 

 carried on into the adult ; in the Asteroidea and Echinoidea the 

 larval mouth and oesophagus are abolished, and a new permanent 

 mouth and oesophagus formed as a fresh invagination from the 

 surface. In the very limited number of Echinoderms that are 

 viviparous there is no such marked metamorphosis ; but even in 

 these the larva is at first distinctly bilateral in its symmetry. 



Ethology, etc. The Echinodermata are, without exception, 

 inhabitants of the sea. In the adult condition the majority 

 creep on the sea-shore or on the sea-bottom, the stalked Crinoids 

 being exceptional in their permanently attached condition ; but 

 the larvae of the great majority Q^Q pelagic i.e. live swimming in 

 the upper strata of the ocean. 



Echinoderms inhabit all depths of the sea, ranging from the 

 shore between low- and high-water limits to the greatest depths. 

 Members of all the classes are found at all depths ; but the stalked 

 Crinoids, and the Elasipoda among the Holothuroidea are virtually 

 confined to the deepest waters of the ocean, only one genus of the 

 former and one species of the latter occurring in comparatively 

 shallow water. Echinoderms are found in the seas of all parts of 

 the globe. Like the majority of marine invertebrate groups, the 

 phylum is more abundantly represented, as regards the number of 

 genera and species, as well as of individuals, in the warmer regions ; 

 the Crinoidea, the Holothuroidea and the Echinoidea are all much 

 more abundant in tropical and warm temperate seas than in colder 

 latitudes. 



Echinoderms are of gregarious habits, large numbers of the 

 same species frequently being found closely associated together in 

 a comparatively narrow area. The movement of locomotion in 

 the Starfishes is, as previously described (p. 348), a slow creeping 

 one through the agency of the tube-feet : the same holds good of 

 the Echinoidea and those of the Holothuroidea (Pedata) that possess 

 tube-feet. The footless Holothurians (Apoda, such as Syriapta) 

 creep along with the help of the tentacles. Most of the Ophiuroids 

 move by lateral flexions, sometimes sluggish, sometimes remark- 

 ably rapid, of the arms. The Comatula3, on the other hand, swim 

 along by the dorso-ventral flexion and extension of the pinnate 

 arms propelling them through the water. Many Asteroids, Ophiu- 

 roids, and Echinoids bury themselves in sand or mud ; others creep 

 into naiTOw fissures in rock or coral. Movements of manducation 

 are performed by the tentacles in the Holothurians : in the Star- 

 fishes the mouth papillae are separated from one another and the 

 cardiac part of the stomach everted in order to enfold the prey, 

 often of relatively large size. In those Echinoidea that possess a 

 lantern of Aristotle there are very powerful and efficient move- 

 ments of mastication. On the whole, as might be expected from 

 the comparatively highly developed muscular and nervous systems. 



