ix PHYLUM ECHINODERMATA 425 



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 are 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. 370), a slow creeping 

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

 the Echinoidea and those of the Holothuroidea that possess tube-feet 

 (Pedata). The footless Holothurians (Apoda, such as Synapta) 

 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 Comatulae, on the other hand, swim 

 along by the flexion and extension of the pinnate arms propelling 

 them through the water. Many Asteroids, Ophiuroids, and 

 Echinoids bury themselves in sand or mud ; others creep into 

 narrow 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, 

 the co-ordination of movement is very much more complete in the 

 Echinodermata than in the groups already dealt with. 



A remarkable characteristic of the Echinoderms is the faculty 

 of self-mutilation which many of them possess, together with the 

 capacity for replacing parts lost in this way or by accidental 

 injury. This is most marked in many Ophiuroids, some Asteroids, 

 and some Holothurians, and does not occur at all among the 

 Echinoids. Many Brittle-stars and some Starfishes, when removed 

 from Mif \\.-if PI, or when molested in any way, break off portions of 



