CHAPTER XVIII 



THE PHYLUM ECHINODERMATA 



Coelomate animals; bilaterally symmetrical as larvae, radially sym- 

 metrical as adults ; whose dermis contains calcareous ossicles ; whose 

 coelom in the larva consists of three segments, and in the adult forms 

 a perivisceral cavity and several intricate systems of spaces, one of 

 the latter being a water vascular system which pushes out the surface 

 of the body as a series of delicate tentacles, the podia or tube feet ; 

 whose vascular system is represented by strands of lacunar tissue; 

 whose principal nervous system remains in contact with the ectoderm 

 from which it arose (though it may be invaginated with the latter) ; 

 which have no nephridia; and whose gonads discharge direct to the 

 exterior by special ducts. 



The group includes the animals familiarly known as starfishes 

 (Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea), 

 sea cucumbers or trepangs (Holothuroidea), and sea lilies (Crinoidea) 



(Fig. 435)- 



The great unlikeness between these animals and all other coelomata 

 is chiefly due to the radial symmetry which they assume at meta- 

 morphosis and which distorts all their systems of organs to its own 

 mould. The radii, which are nearly always five in number, diverge 

 from the mouth. The surface of the body upon which the mouth lies 

 is known as the oral or ambulacral, the opposite surface as the aboral 

 or abambulacral. The terms ''ventral" and "dorsal" should not be 

 applied to these surfaces, for they correspond not to the ventral and 

 dorsal but to the left and right sides of the larva. The anus, if present, 

 lies usually on the aboral side, but in the Crinoidea it lies on the oral 

 side. The alimentary canal runs a straight or devious course from 

 mouth to anus. The other systems consist each of a ring around the 

 axis which passes through the mouth and the middle of the aboral 

 side, and a tube or cord along each radius. The radii are constituted 

 by the presence of the radial members of the various systems. The 

 areas between the radii are known as interradii. Most of the systems 

 lie close under the ambulacral surface, and the tube feet project from 

 it, forming radial bands known as the ambulacra. In the Asteroidea 

 and Crinoidea the tube feet of each ambulacrum stand on either side 

 of an ambulacral groove at the bottom of which lies the highly nervous 

 strip of epithelium which is the radial "nerve cord". In the other 

 classes the ambulacral groove is roofed in, forming an epineural canal 

 over the nerve cord. In the Asteroidea, Ophiuroidea and Crinoidea 



