the Crinoiclea, Cystidea, and Blastoidea. 415 



connected with any other part. But when, as in this instance, 

 we can actually see that it is an appendage of another organ 

 (or system of organs, rather), Avhicli is known to be the liomo- 

 logue of the part with which that of the existing species is 

 always correlated, we have evidence of a very high order on 

 which to ground a conclusion. By no other mode of reasoning 

 can we prove that the column of an Actinocrinus is the homo- 

 logue of that of Pentacrinus caput Medusa;. 



In an important paper entitled " Remarks on the Blastoidea, 

 with Descriptions of New Species," which Messrs. Meek and 

 Worthen have kindly sent me, the authors, in their comments 

 uj^on my views, state that — 



" In regard to the internal convoluted organ seen in so many 

 of the Actinocrinidffi, belonging to the respiratory instead of the 

 digestive system, we would remark that its large size seems to 

 us a strong objection to such a conclusion. In many instances 

 it so nearly fills the whole internal cavity that there would 

 appear to be entirely inadequate space left for an organ like a 

 digestive sac outside of it, while the volutions within would 

 preclude the presence of an independent digestive sac there. 

 In addition to this, the entire absence, so far as we can ascer- 

 tain, of any analogous internal respiratory organ in the whole 

 range of the recent Echinodermata, including the existing 

 Crinoids, would appear to be against the conclusion that this 

 is such, unless we adopt the conclusion of Dujardin and Hupe, 

 that the paleozoic Crinoids had no internal digestive organs, 

 and were nourished by absorption over the whole surface. 

 We should certainly think it far more probable that this spiral 

 organ is the digestive sac than a part of a respiratory appa- 

 ratus." 



The objection here advanced does not appear to me to be a 

 strong one. In many of the lower animals the digestive organs 

 are of inconsiderable size in proportion to the whole bulk. In 

 the Brachiopoda, for instance, the spiral ciliated arms fill nearly 

 the whole of the internal cavity, the digestive sac being very 

 small and occupying only a limited space near the hinge. 

 These arms, although not the homologues of the convoluted 

 plates of the palasozoic Crinoids, have a strong resemblance to 

 them, and are, moreover, at least to some extent, subservient 

 to respiration. They are certainly not digestive sacs. In the 

 recent Echinoderms the intestine is usually a slender tube 

 with one or more curves between the mouth and the anus. It 

 fills onlya small part of the cavity of the body, the remainder 

 being occupied mostly by the chylaqueous fluid, which is con- 

 stantly in motion and undergoing aeration through the agency 

 of various organs, such as the respiratory tree and branchial 



