432 THE CANADIAN NATURALIST. [DeC. 



poric sacks and tubes (or sand canals) of the recent Echinoderms, 

 therefore, all agree in the following respects: — 



1. They have the same general structure. 



2. They are all appendages of the ambulacral system. 



3. They are all attached to the same part of the system, that 

 is to say, to the central point from which the canals radiate. 



The above seems to me sufficient to make out at least a good 

 jjnWf/acie case for the position I have assumed. When among 

 the petrified remains of an extinct animal, we find an organ 

 which has the same general form and structure, as has one that 

 occurs in an existing species of the same zoological group, we 

 may, with much probability of being correct in our opinion, con- 

 clude that the two are homologous, even although we may not be 

 able positively to see how that of the fossil is 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, which is known to be the homologue 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 homologue of that of Pentacrinus caput 

 Medusce. 



In an important paper, entitled "Kemarks on the Blastoidea, 

 with descriptions of New Species," which Meek and Worthen have 

 kindly sent me, the authors, in their comments upon my views, 

 state that : — 



" In rejiard to the internal convohited organ seen in so many of the 

 Acti)iocrld(ie belongiag to the respiratory instead of the digestive system, 

 we woul i remark that its large size seems to us a strong objection to 

 such a conclusion. In many instances it so neai-ly fills the whole internal 

 cavity that there would appear to be entirely inadequate space left for 

 an organ like a digestive sack, outside of it, while the volutions within 

 would preclude the presence of an independent digestive sack there. 

 In additon to this, the entire absence, so far as we can ascertain, of any 

 auaU)g()us, 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 Palfeozoic Oi-inoids 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 sack, than a part of a respiratory apparatus." 



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

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



