1869.] BILLINGS — ON THE STRUCTURE OF CRINOIDEA, ETC. 433 



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

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

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

 small and occupying only 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 

 intestine 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 various organs, such as the respiratory tree 

 and branchial cirrhi of the Holothuridea, the dorsal tubuli of the 

 Asteridae and the anibulacral systems of canals of the class gene- 

 rally. 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 favour 

 of its being a respiratory then a digestive organ. 



Prof. Wyville Thomson says, that inside of the cavity of the 

 stomach of the recent Crinoid, Antedon I'osaceus, 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 why 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. 



(7b he continued.') 



