410 ZOOLOGY sect. 



stalked stage known as the " pentacrinoid" larva on account of the 

 resemblance which it bears to the adult Pentacrinus, one 

 of the permanently fixed members of the same class. This fixed 

 pentacrinoid larva passes into the adult free-swimming Feather- 

 star by the development of the dorsal cirri, the elongation of the 

 arms, and the absorption of the stalk. 



5. Distinctive Characters and Classification. 



The Echinodermata are radially symmetrical animals, the radial 

 arrangement of whose parts imperfectly conceals a more obscure 

 bilateral symmetry. The surface is covered with an exoskeleton 

 of calcareous plates or ossicles, which usually support a system of 

 movable or immovable calcareous spines. There is a large body- 

 cavity or ccelome, and well-developed alimentary, nervous, and 

 vascular systems. A characteristic system of vessels, the ambu- 

 lacral system, is connected with the locomotion of the animal, as 

 well as with other functions : the organs of locomotion are in most 

 cases elastic and contractile tubular bodies, the tube-feet, which are 

 appendages of the ambulacral system. Nearly all the systems of 

 organs of the animal partake to a greater or less extent of the 

 general radial form of the body. Reproduction is entirely sexual. 

 In the course of its development from the egg the Echinoderm 

 passes through a peculiar larval stage, in which the symmetry of 

 parts is bilateral, instead of radial as in the adult animal. All the 

 Echinodermata are marine. 



The Echinodermata are classified as follows : — 



SUB-PHYLUM I.— ELEUTHER0Z0A. 



Echinodermata devoid of a stalk, and always freely locomotive in 

 the adult condition : with a system of radial ambulacra in the form 

 of grooves or areas radiating out from the mouth, and containing 

 a double series of tubular appendages of the ambulacral system, 

 the tube-feet, usually employed in locomotion, and in the majority 

 of cases provided with terminal suckers : the anus usually aboral ; 

 the mouth on the surface that is habitually directed downwards, or 

 at the end habitually directed forwards in locomotion. 



CLASS I.— ASTEROIDEA. 



Free Echinoderms with star-shaped or pentagonal body, in 

 which a central disc and usually five arms are more or less readily 

 distinguishable, the arms being hollow, and each containing a 

 prolongation of the ccelome and of its contained organs. There 

 arc distinct oral and aboral surfaces, on the former of which the 

 anus and the madreporite are situated, and on the latter the 



