Introduction to Animal Morphology. 149 



system. The anus has a round or pentagonal muscular 

 sphincter, into whose edges the longitudinal body 

 muscles are inserted. The pharynx is oval and 

 glandular, separated from the stomach by a sphincter, 

 surrounded at its origin by a moveable ring of 10, 12, or 

 15 plates, five of which are usually notched or pierced 

 (Sj^napta), radial, and five are inter-radial ; these re- 

 present the auricles of Echini. Five muscular bands 

 unite these plates, and 10-12 pyramidal bands arise 

 by their apices therefrom, and have their broad bases 

 inserted into the pharyngeal wall : they can draw 

 forwards the pharynx. The digestive canal is straight 

 or curved, attached by often uninterrupted mesenteries. 

 It sometimes ends in a cloaca directly above the anus. 

 In the intestine there may be 2-4 longitudinal folds, 

 possibly the extension of an absorbing apparatus. 

 The alimentary canal contains diatoms, foraminifers, 

 and sand. 



The dermis acts as a respiratory organ, so do the 

 remarkable tree-like, long branching organs, ciliated 

 within and without, which arise from the cloaca, and 

 stretch in the body cavity towards the mouth. Water 

 enters them from the cloaca, and passes into the body 

 cavity from them by holes in the tips of the branches. 

 These water-lungs are homologous to the inter-radial 

 caeca of Asteriadae. Water enters in other forms by 

 inter-tentacular pores. The tree-like organs vary in 

 size; in some forms they are joined to the wall of the 

 somatic cavity by threads, and even where the trees 

 are absent (Synapta), these threads remain appended 

 to the mesenteries with attached to their inner ends 

 ciliated, slipper-like, or cornucopiae-like organs, and 

 communicating with the pseud-haemal system. Rarely 



