109 



other extremity hanging freely down into the perivisceral cavity, not 

 connected with the opposite body wall as is the sand canal of the star- 

 fishes. (Op. cit., p. 84.) In its consisting of a convoluted plate the 

 madreporic organ of ActinocrhiKS, therefore, agrees with that of the star- 

 fishes, while in its being only attached at one extremity it resembles that 

 of the Holothurians. 



The convoluted plate of the Palaeozoic Crinoids and the madreporic 

 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 prima 

 facie case for the position I have assumed. When among the petrified 

 remains of an extinct animal, we find an organ which has the same gene- 

 ral 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 m 

 our opinion, conclude 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 

 Fentacrinus caput Mediisce, 



In an important paper entitled Remarks 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 regard to the internal convoluted organ seen in so many of the Actinocrinidi'e h^loug^ 

 ing to the respiratory instead of the digestive system, we would remark that its lavge 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 yiadequate 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 addition to this, the entire 

 absence, so far as we can ascertain, 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 Dujardiii 

 and Hupe, that the Palseozic Crinoids had no internal digestive organs, and were nour- 

 ished by absorption over the whole surface. We should certainly think it far more proba- 

 ble 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. 



