46 ON CLASSIFICATION. 



with the exterior is doubtful, aud still more so in Crinoids. 

 In Holothurids no such communication obtains, the madreporic 

 canals and their tubercles depending freely from the circular 

 canal into the perivisceral cavity. 



Whether the larva possessed a skeleton or not, the adult 

 Echinoderm presents a calcareous framework which is developed 

 quite independently of that of the larva. This skeleton may 

 be composed of mere detached spicula, or plates, as in the 

 Holothurids; or of definitely disposed ossicula, or regular 

 plates, as in other Echinoderms. In the latter case its parts 

 are always disposed with a certain reference to the disposition 

 of the ambulacral svstem, and hence have a more or less dis- 

 tinctly radiate arrangement. It might be expected, in fact, 

 that the arrangement of the organs of support should follow 

 more or less closely that of the chief organs of movement of the 

 adult Echinoderm, and it is not surprising to find the nervous 

 system similarly related. It is, in all adult Echinoderms, a 

 ring-like, or polygonal, ganglionated cord, situated superficially 

 to that part of the ambulacral system which surrounds the 

 mouth, and sending prolongations parallel with, and superficial 

 to, the radiating ambulacral trunks. 



The reproductive organs of the Echinoderms, which usually 

 open upon, or between, parts of the radially disposed skeleton, 

 commonly partake of the radial symmetry of that skeleton ; but 

 they have no such radial symmetry in the Holothuridea. 



The alimentary canal of the adult Echinoderm is still less 

 dependent upon the skeleton, and only in one group, the 

 Asteridea, exhibits anything approaching a radiate disposition. 

 ^Yhere skeletal elements are developed around the mouth or 

 gullet, however, they have a radial disposition ; as, e. g., the parts 

 of the so-called " lantern of Aristotle." 



The vascular system which exists in many, if not all, adult 

 Echinoderms, but the true nature of which is by no means 

 understood at present, is closely related both to the alimentary 

 and to the ambulacral systems, and partakes of the disposition 

 of both. 



No Echinoderm whatsoever has its organs, internal or ex- 

 ternal, disposed with that absolute and perfect radial symmetry 



