VI. BRACHIOPODA 



289 



The usually spirally coiled arms, which lie right and left of the mouth 

 and which give the name to the class, ilU most of the shell. On the outer 

 side of each arm is a longitudinal groove, bounded by a row of small ten- 

 tacles. By means of cilia on tentacles and groove food is brought to the 

 mouth. These arms resemble the lophophore of a phylactokcmate Poly- 

 zoan, which only needs extension and coiling to produce this condition. 

 In development the arms of the Brachiopod pass through a lophophore 

 stage. 



In the body there is a ciliated coclom which extends into both arms and 

 mantle folds. It encloses alimentary tract, gonads, and liver, and is 



Fig. 278. — Waldheiniia flavescens (from Zittel). A, dorsal, B, ventral valve; a, 

 h, f, impressions of muscular insertions; a, adductors; h", adjusters (stalk muscles); 

 c, c', divaricators; s, hinge groove of upper valve in which the tooth (/) of the lower 

 valve passes; I, support of arms; d, deltidium;/, foramen for stalk. 



divided into right and left halves by dorsal and ventral mesenteries sup- 

 porting the intestine. Each half in turn is divided by incomplete septa into 

 anterior, middle, and posterior divisions recalling those olSagitta (p. 252). 

 The arrangement of the septa is not so clear as in that form, the result of 

 the shortening of the long axis and the twisting of the alimentary tract. 

 This latter consists of oesophagus, stomach, which receives the liver ducts, 

 and intestine, which in some species terminates blindly. 



The gonads are chiefly in the mantle lobes. The sexual cells pass out 

 through the nephridia, which begin in one coelomic pouch with wide 

 nephrostomata, perforate the septum, and open to the exterior in the next 

 somite. Since usually there are two septa, two pairs of nephridia may 

 occur, but one is usually degenerate. The nervous system consists of an 

 oesophageal ring with weak dorsal ganglion, extending into the arms, and a 

 stronger ventral mass representing the ventral chain. The heart lies 

 dorsal to the stomach. 



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