378 ZOOLOGY SECT. 



alimentary, nervous, and vascular systems. A characteristic 

 system of vessels, the ambulacral system, is connected with the 

 locomotion of the animal, as well as with other functions : the 

 organs of locomotion are 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. Repro- 

 duction 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 : 



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 coelome and of the contained organs. There 

 are distinct dorsal and ventral surfaces, on the former of which 

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

 mouth and five narrow ambulacral grooves lodging the tube-feet. 

 The larva has the form .either of a Bipinnaria or of a Brachiolaria 

 (see p. 397). This class includes the Starfishes. 



ORDER 1. PHANEROZONIA. 



Asteroidea with large marginal ossicles. The dermal branchiae 

 are present only on the dorsal surface. The ambulacral ossicles 

 not closely crowded. PedicellariaB sessile. 



ORDER 2. CRYPTOZONIA. 



Asteroidea with the marginal ossicles inconspicuous. Dermal 

 branchiae not restricted to the dorsal, but often present on the 

 oral surface. Ambulacral ossicles crowded together. Pedicellariae 

 stalked or sessile. 



CLASS II. OPHIUROIDEA. 



Star-shaped free Echinoderms, with a central disc and five arms, 

 which are more sharply marked off from the disc than in the 

 Asteroidea, and which contain no spacious prolongations of the 

 coelome. There are distinct dorsal and ventral surfaces. The 

 anus is absent ; the mouth, as well as the madreporite, ventral. 

 There are no ambulacral grooves. The larva is a Plutcus. This 

 class includes the Sand-stars and Brittle-stars. 



ORDER 1. OPHIURIDA. 

 Ophiuroidea in which the arms are simple. 



ORDER 2. EURYALIDA. 



Ophiuroidea in which the arms are branch od. 



