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Subkingdom III. ECHINODERMATA. 



Marine animals, widely different in appearance, but with more 

 or less of a radiate structure. An alimentary canal distinct from 

 the body-cavity, and mostly with an anal aperture. A vascular 

 system in most. Deuterostomatous. Sexes mostly distinct. 



The egg generally develops a free-swimming, ovoid, ciliated, 

 bilateral larva or pseudo-embryo, called a "Pluteiis" A process 

 of modification or secondary development within the embryo or 

 larva, and absorbing its digestive organs, leads to the adult form ; 

 the parts of the larva therefore do not correspond with those of 

 the adult, they are either absorbed or cast off. There is, however, 

 often a direct development, the young being reared within or 

 upon the body of the parent, and retaining a commensal relation 

 with her until able to provide for themselves. 



All Echinoderms have a calcareous skeleton, and many are pro- 

 vided with movable spines. A characteristic apparatus of vessels, 

 termed the ambulacral or water-vascular system, is present. It 

 is composed of a ring round the pharynx, from which proceed a 

 number of radiating canals, commonly giving off caecal appen- 

 dages [Polian vesicles] as well as branches, which enter the re- 

 tractile tube-feet, often furnished with a terminaLdisk or sucker 

 [ambulacra], which, with the spines, are the organs of locomo- 

 tion. The " madreporic canal " connects the pharyngeal ring 

 with the exterior. " The ambulacral vessels are filled with a fluid 

 containing numerous nucleated cells." 



Originally described as parasitic animals are the " pedicellarise " 

 (homologous with the spines, according to A. Agassiz), found in 

 some members of the Echinoidea and Asteroidea. They are small 

 slender bodies having a soft skin, with two or three claws at the 

 top, which they open and shut with great activity ; their use is 

 supposed to be for cleansing purposes. 



The Echinodermata, are said to have no annectant forms. The 

 Gephyrea resemble certain Holothurioidea ; but the structural dif- 

 ference between their larvae are such that they "can never be 

 genealogically connected." A. Agassiz, however, considers that 

 there is " nothing in nature to justify their separation " as a sub- 

 kingdom from Ccelenterata (1877). 



There are four classes ; but the Crinoidea are sometimes in- 

 cluded in the Stellerida. 



Body stalked CRINOIDEA. 



Body not stalked. 

 An external shell of calcareous plates. ECHINOIDEA. 



