156 



CTENOPIIOR.E. 



Part II. 



The Ctenopliura) are free Acaleplis inoving in various ways, tlieir main axis 

 being generally turned in the ilireetion of their onward motion, Ijut at times also 



valion (if mine, tliat K. E. von I!;kt long ago 

 already insisted ui)on the necessity of the distinction, 

 in view of an aeenrate a|i[H'eeiation of the trne re- 

 lations of animals, though his warning has not 

 been Iieeded. (Comp. vol. 1, p. 221.) The reason 

 why Baer's suggestion to distinguish between the 

 detjree of jyerfection in the structure of animals and 

 the tijpe of organization has not been followed 

 out, lies perhaps in the vagueness of the ex- 

 pressions he uses : l)ut whosoever has compre- 

 hended that distinction for himself cannot fail to 

 ])ereeive that what I have alluded to when dis- 

 cussing tlie value and significance of the phius of 

 structure of the animal kingdom with reference to 

 classification, is the same tiling as what Eaer calls 

 tlie type of orffani:a/iun ; and that wliat he calls 

 the degree of perfection in the structure of animals 

 corresponds to the two features of tlicir structure 

 which I have distinguished as modes of execution 

 and degrees of complication, after I had perceived 

 that r>aer confounds under one ex))rcssion two 

 distinct categories of structure, — inw relating, indeed, 

 to the relalire degrees of perfection in the animal 

 structure ujion wliich firdcrs are founded, lait 

 ]iot necessarily including anotiier, broaclcr con- 

 sideration, the wags and means of the execution 

 of the plan, upon which alone classes are based. 

 (Comp. vol. 1. p. KJ7-17G). To some extent 1 

 have already ]iointed out the general homologies 

 which unite the Echinoderms and the delenterata 

 (pp. Gl-87, and pji. 00-1 1;3); but it is so ditiicult 

 to trace these general comparisons througli the 

 obstructions of a confused nomenclature, and in the 

 face of the still greater obstacles arising from the 

 remoteness of the whole type of Radiates from 

 that to which we ourselves belong, that a thorougli 

 appreciation of the general as well as the special 

 homologies of these animals can only be the result 

 of a prolonged comparative stud)- of all their lyjies, 

 and an equal familiarity with all of them. I 

 venture to say, that if Leuckart and Gegenbaur 

 had devoted tlieir special attention to the Echi- 



noderms as extensively as to the Acalephs, they 

 would feel less confident that there is a typical 

 diflVrence between them. As for myself, I must 

 declare, in the words of K. E. von Baer, that I 

 can perceive only " diflTerent degrees of perfection 

 in their structure," and no difference "in the type 

 of their organization ; " or, in the words of my 

 Essay on Classification, Polyps, Acalephs, and Echi- 

 noderms are built upon "one and the same plan 

 of stnu'ture," and therefore belong to the same 

 branch of the animal kingdom, while as classes 

 they differ in the " modes of execution of that 

 plan." As classes of one branch, they are held 

 together by general homologies, while special homolo- 

 gies may be traced respectively in all the repre- 

 sentatives of these classes. The most striking of 

 these general homologies, because thus far least 

 noticed, unquestionably, is that of the aquiferous 

 system of the Echinoderms, and the radiating 

 chambers of the Polyps, linked together by the 

 cln'iniferous tulies of the Acalejihs. It is not my 

 intention here to trace all these homdlogies, to 

 which I shall devote a special chapter in the 

 secpiel ; but, since the appreciation of the true 

 relations of the Echinoderms to the other Radiates 

 must di'pend upon the views entertained of their 

 homologies, I would urge ujion the natur.alists who 

 Consider the Echinoderms as a distinct tyjie, the 

 importance of closely comparing on one side the 

 sim[)ler ambulacral system of the lower llolothurians 

 with llie radiating chymiferous tubes of the naked- 

 eyed Jledusa', and on the other side the peculiar 

 mode of brandling of the chymiferous tubes in the 

 genus Aurelia with the ramifications of the aquiferous 

 system in .Scutella and Echinarachniiis ; or the 

 radiating pouches of Cyanea, and their numerous 

 tentacles opening freely into these cavities, with the 

 amliulacral suckers of any Star-fisli ; or the circular 

 aquiferous tube of Echinarachnius with the cii-cular 

 chymiferous tub(? of the naked-eyed Medusiv, — and 

 I doubt not that the result of such comparisons will 

 be a growing conviction, that tlie spherosome of the 



