110 



In many of the lower animals the digestiv^e organs are of inconsiderable 

 size in proportion to the whole bulk. In the Brachiopoda, for instance, 

 the spiral ciUated arms fill nearly the whole of the internal cavity, the 

 digestive sack being very small and occupying only a limited space near 

 the hinge. These arms, although not the homologues of the convoluted 

 plates of the Palaeozoic Crinoids, have a strong resemblance to them, and 

 are, moreover, at least to some extent, subservient to respiration. They 

 are certainly not digestive sacks. In the recent echinoderms the intes- 

 tme is usually a slender tube with one or more curves between the mouth 

 and the anus. It fills only a small part of the cavity of the body, the 

 remainder being occupied mostly by the chylaqueous fluid, which is 

 constantly in motion and undergoing aeration, through the agency of vari- 

 ous organs, such as the respiratory tree and branchial cirrhi of the Holo 

 thuridea, the dorsal tubuli of the Asteridas and the ambulacral systems of 

 canals of the class generally. In no division of the animal kingdom do 

 the respiratory organs occupy a larger proportion of the whole bulk than 

 they do in the Echinodermata. The great size which the convoluted plate 

 attains in some of the Crinoids is, therefore, rather more in favor of its 

 being a' respiratory than a digestive organ. 



Professor Wyville Thomson says that inside of the cavity of the stom- 

 ach of the recent Crinoid, Antedon rosaceus, there is a spiral series of 

 glandular folds which he supposes to be a rudimentary liver. (Phil. 

 Trans. R. S., 1865, p. 525). It is barely possible that the convoluted 

 plate may represent this organ. At present I think it does not. 



I believe that the reason w^hy the convoluted plate attained a greater 

 proportional size in the Palaeozoic Crinoids, than do the sand canals of 

 the recent echinoderms, is that the function of the system of canals (of 

 which they are all appendages,) was at first mostly respiratory, whereas 

 in the greater number of the existing groups, it is more or less prehensive 

 or locomotive, or both. 



6. 0)1 some ]}oints relating to the structure ofPentremites. 



Professor Wyville Thompson has proposed a division of the skeleton of 

 the existing Crinoid, Antedon rosacetis, into two systems of plates, which 

 he terms respectively the " Badial,^^ and the Perisomatic^^ systems.* 

 These he considers to be thoroughly distinct from each other in their struc" 

 ture and mode of growth. The radial system consists of the joints of the 

 stem, the centrodorsal plate, the radial plates, the joints of the arms, and 

 also those of the pinnules. In the perisomatic system he includes the 



* On the Embryogeny of Antedon rosaceus Linck (Comatula rosacea of Lamarck.) By 

 Professor Wyyille Thompson, LL.D., &c, Philosophical Transactions of the Royal Socie- 

 ty, Tol. cly, Part II, p. 540. 



