viii ECHINODERMATA RESPIRATORY ORGANS 485 



merit, and size are subject not only to specific arid generic, but often 

 also to individual variations, are, as a rule, much smaller than the 

 brachial diverticula of the stomach, with which they otherwise agree 

 in structure. 



The anus is wanting only in the Astropectinidce. Elsewhere it 

 lies somewhat eccentrically (never exactly at the apical pole) in the 

 interradius which follows the madreporitic interradius (in the direction 

 of the hands of the clock), when the disc is seen from the apical 

 side. 



Finer structure of the intestine. The intestinal epithelium is ciliated. Glandular 

 cells, as goblet cells, mucus cells, and granular cells, are everywhere found together 

 with epithelial cells. The last named appear to secrete especially the digestive 

 ferment ; they preponderate at the commencement and terminal part of the canal, 

 and are particularly numerous in the brachial and rectal diverticula. The muscle 

 layer is well developed in the oesophagus, the rectum and the rectal diverticula, less 

 strong in the stomach, and wanting in the brachial diverticula. 



The manner in which the brachial diverticula are suspended to the apical brachial 

 wall has already been described (p. 440). 



The Asteroidea are carnivorous, feeding on other marine animals, especially 

 Bivalves and Gastropods. When feeding, they evaginate the greater part of the 

 stomach out of the oral aperture, enveloping their prey with it. The secretion of 

 the mucus cells yielded during the process appears to be poisonous and to have a 

 decomposing action. The animals attacked quickly die, and are passed on to the 

 part of the stomach still remaining within the disc, where they undergo the digesting 

 action of the secretion yielded by the granular glands (Kornerdriisen). 



The evagination of the stomach is brought about by the musculature of the disc, 

 and its withdrawal by the (partially) muscular mesenterial bands which attach it to 

 the body wall. 



The anal aperture certainly does not serve for the ejection of all the fecal masses. 

 It is impossible that large masses, such as the shells of Bivalves and Gastropods, 

 which are found in the stomachs of Asteroids, can be ejected through such a narrow 

 aperture ; they are no doubt passed out again through the mouth. 



F. Ophiuroidea (Figs. 386 and 388, p. 494). 



The condition of the alimentary canal in this class is simpler than 

 in any of the others. The somewhat spacious buccal cavity which is 

 surrounded by the oral skeleton leads into the digestive sac which 

 fills the body cavity of the disc, in so far as it is not occupied by the 

 bursse. An anus is wanting. Special intestinal appendages in an}^ 

 way corresponding with the brachial or rectal diverticula of the 

 Asteroids are wanting. 



XV. Respiratory Organs. 



There are no respiratory organs which are homologous throughout 

 the whole phylum of the Echinodermata. Portions of the body 

 belonging to very different organs and systems of organs are function- 



