XVII THE ALIMENTARY CANAL 47 1 



protrude its arms to a distance of 4 cm. beyond the anterior 

 borders of the shell, a distance nearly equalling twice the length 

 of the shell." The same observer also mentions that Lingula 

 has the power of partially protruding its arms. In most genera 

 the cirrhi or tentacles can alone be protruded. 



The cilia which clothe the tentacles keep up a constant flow 

 of water into the mantle cavity. This stream not only serves 

 to aerate the blood of the animals — a process which probably 

 takes place through the thin inner lining of the mantle — but it 

 also brings with it a number of diatoms and other minute 

 organisms which serve as food. These particles become en- 

 tangled in the tentacles, and are ultimately lodged in the groove 

 at their base, and passing along this by the action of the cilia 

 they find their way into the wide mouth, into which the groove 

 deepens in the posterior median line. 



The Digestive System 



The mouth leads into an oesophagus; this widens into a 

 chamber which may be termed the stomach (Fig. 314), and which 

 receives the openings of two large branching glands usually 

 known as the liver. The stomach passes into a short intestine 

 which is usually bent at about a right angle with the oesophagus. 

 In the Testicardines the intestine ends blindly, but in the 

 Ecardines it is of much greater length, and terminates in an 

 anus, situated posteriorly in the median line in Crania^ but 

 asymmetrical and to the right of the body in Lingula (Fig. 315) 

 and Discina ; in both cases, however, the opening is into a por- 

 tion of the mantle cavity. The alimentary canal is supported 

 by a median dorsal and ventral mesentery, and by two pairs of 

 lateral mesenteries which pass to the body wall. The lateral 

 mesenteries are not always quite distinct. When they are, the 

 anterior pair are known as the gastro-parietal bands, and the 

 posterior as the ileo-parietal. In Rhynclionella there are two 

 pairs of renal organs, and each of these mesenteries bears the 

 internal openings of one pair. In all other Brachiopods there is 

 only one pair, and they are supported by the ileo-parietal bands. 



The alimentary canal is ciliated throughout, and some inter- 

 esting observations have been made by Schulgin^ on the shorten- 



1 Loc. cit. p. 470. 



