CHAPTER XVIII 



THE PHYLUM ECHINODERMATA 



Coeloraate 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. 410). 



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 

 abamhulacral. 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 



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