STRUCTURE OF BRACUIOPODS. 189 



composed of carbonate (Terebratulina) or largely (Lingula, 

 Fig. 133) of phosphate of lime. It is really the thickened 

 integument of the animal, the so-called mantle being the 

 inner portion of the skin, containing minute tubular canals 

 which do not open externally. 



The body of Brachiopods is divided into two parts, the 

 anterior or thoracic, comprising the main body-cavity in 

 which the arms and viscera are contained, and the caudal 

 portion, i. e. the peduncle. The part of the body in which 

 the viscera lodge is rather small in proportion to the entire 

 animal, the interior of the shell being lined with two broad 

 lobes, the free edges of which are thickened and bear setae, 

 as seen distinctly in Linyula. The body-cavity is closed 

 anteriorly by a membrane which separates it from the space 

 in which the arms are coiled up The "pallial cham- 

 ber" is situated between the two lobes of the mantle (pal- 

 lium) and in front of the membrane forming the anterior 

 wall of the body-cavity. In the middle of this pallial 

 chamber the mouth opens, bounded on each side by the 

 base of the arms. The latter arise from a cartilaginous 

 base, and bear ciliated tentacles, much as in the worm Sa- 

 l)dla. In Lingula, Discina, and Rliynchomlla, they are de- 

 veloped, as stated by Morse, in a closely-wound spiral, as in 

 the genuine worms (Amphitrite). In Lingula the arms can 

 be partially unwound, while in Rhynchonella they can not 

 only be unwound but protruded from the pallial chamber. 

 In many recent and fossil forms the arms are supported by 

 loop-like solid processes of the dorsal valve of the shell, but 

 when these processes are present the arms cannot be pro- 

 truded beyond the shell. The tentacles or cirri on the arms 

 are used to convey to the mouth particles of food, and they 

 also are respiratory in function, there being a rapid circula- 

 tion of blood in each tentacle, which is hollow, communi- 

 cating with the blood-sinus or hollow in each arm, the sinus 

 ending in a sac on each side of the mouth. 



The digestive system consists of a mouth, oesophagus, 

 stomach, with a liver-mass on each side, and an intestine. 

 Fig. 128 shows the relation of the mouth and digestive canal 

 to the head and arms, as seen in a longitudinal section of 



