46 INTRODUCTION TO CLASSIFICATION. 



with the exterior is doubtful, and 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 Holo- 

 thurids ; 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 system, and hence have a more or less distinctly 

 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 

 Echinoderra, 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. 

 Where 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 



