146 WYVILLE THOMSON, ON SYNAPTA INHiERENS. 



but with a provisional alimentary canal more rudimentary 

 than that of an Auricularia, is developed at once from the 

 ciliated germ ; and sometimes after only a few hours of inde- 

 pendent life this zooid passes, by simple metamorphosis, into 

 a stalked Crinoid. 



It is impossible to doubt that the tentacles of the Holothu- 

 ridse, in whatever form they may be developed, are the equi- 

 valents of the minute oral tentacles of the Echinidae, and it 

 is equally evident that the ambulacra of a Cucumaria or of a 

 Holothuria are homologous with the ambulacra of an 

 Echinus. 



As in Echinus, then, according toMliller's view, nearly the 

 whole of the body in Holothuria must be regarded as am- 

 bulacral; the antambulacral region being restricted to a small 

 disc immediately within the anal vascular ring. The same 

 relation must hold for Synapta ; for the five longitudinal 

 muscular bands, with their accompanying vessels, must 

 represent the ambulacra of the higher forms, though the 

 A'essel is now reduced to a mere filament, and though the 

 characteristic ambulacral suckers are totally gone. Thus the 

 oral tentacles coexist in Echinus and in Cucumaria with 

 highly developed ambulacra, and in Synapta, with ambulacra 

 represented by mere rudimentary vascular twigs. 



No one who observes the development, the structure, and 

 the mode of action of the tentacles in a genus such as Ocnus, 

 in which these organs are highly developed, and mailed with 

 calcareous plates; can fail to be struck with their resemblance 

 to the arms of Crinoids. I have no doubt whatever that 

 we must consider the pinnae of Comatula as the equivalents 

 of the tentacles of Cucumaria, that there is no reason to 

 regard the arms of Crinoids as free ambulacra, but that the 

 calyx is the true ambulacral region, and that in Comatula and 

 in many of the fossil genera, the ambulacra, as in Synapta, 

 are undeveloped. 



Many of the Cystideans and some Crinoids seem to have 

 possessed both arms and ambulacra ; and there is clearly no 

 reason why they should not, if we look upon the Crinoids as 

 I am inclined to do, as partaking much more of the cha- 

 racter of stalked Holothurids, than of stalked Urchins or 

 Stellerids. I have mentioned above, that the Crinoids 

 approach much more closely in their mode of development to 

 the Holothuridse than to any of the other Echinoderm groups ; 

 and I regard this as a strong additional fact in favour of the 

 close affinity of the two orders. 



