CHAPTER TWENTY-NINE 



Origin and 

 characters 

 of Echino- 

 dermata 



ECHINODERMATA 



I. THE origin of the Echinodermata is problematical, 

 but they are certainly much less primitive than the 

 Ccelenterata. The larva is more or less wormlike or 

 curiously branched, with a distinct bilateral symmetry. 

 There seems to be a certain relationship with the Cirri- 

 pedia or barnacles, and therefore with the Arthropoda. 

 However this may be, the phylum is one of the most dis- 

 tinct and easily recognized, though its different members 

 are very diverse. They inhabit the sea, although one of 

 the wormlike sea cucumbers (Synapta) may be found in 

 brackish water in mangrove swamps. The adult ani- 

 mals are usually recognizable by their radial symmetry, 

 with a calcareous outer skeleton ; internally we find a 

 complete alimentary canal, with two openings, and a 

 body cavity between this and the outer wall. The nerv- 

 ous system is closely connected with the skin, and there 



Draining by W. P. Hay 



FIG. 50. Common starfish (Aslerias) of Atlantic Coast. A, upper or aboral sur- 

 face ; B, lower or oral surface ; C, cross-section of one of the arms ; D, diagram of 

 the water- vascular system; m, madreporic body; e, eye; mo, mouth; ag, ambula- 

 cral groove ; /, tube feet ; rt, radial water tube ; /, digestive gland ; b, body cavity ; 

 j, plates of skeleton. 



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