2/2 MANUAL OF ZOOLOGY. 



acute angle usually only about 10 or 12 with the surfaces 

 of the shell." (Carpenter.) In most cases, also, the shell is 

 perforated by a series of minute canals, which pass from one 

 surface of the shell to the other, in a more or less vertical 

 direction, usually widening as they approach the external 

 surface. These canals give the shell a " punctated " structure, 

 and in the living animal they contain caeca! tubuli, or pro- 

 longations, from the mantle, which are considered by Huxley 

 as analogous to the vascular processes by which in many 

 Ascidians the muscular tunic, or " mantle," is attached to the 

 outer tunic, or " test." In some of the Brachiopoda (as in the 

 Rhynchonellidcz) the shell is " impunctate," or is devoid of this 

 singular canal system. 



The inner surface of the valves of the shell is lined by ex- 

 pansions of the integument which secrete the shell, and are 

 called the " lobes " of the " pallium," or " mantle." The diges- 

 tive organs and muscles occupy a small space near the beak 

 of the shell, which is partitioned off by a membranous septum, 

 which is perforated by the aperture of the mouth. The re- 

 mainder of the cavity of the shell is almost filled by two long 

 oral processes, which are termed the " arms," and from which 

 the name of the class has been derived (fig. 98, i.) These 

 organs are lateral prolongations of the margins of the mouth, 

 usually of great length, closely coiled up, and fringed on one 

 side with lateral processes, or " cirri." In many Brachiopods 

 the arms are supported upon a more or less complicated inter- 

 nal calcareous framework or skeleton, which is sometimes called 

 the " carriage-spring apparatus." 



Fig. 98. Brachiopoda ( Terebratula vitred). i. Showing the ciliated " arms ;" 

 2. Showing the shell with its loop. (After Woodward.) 



The mouth conducts by an oesophagus into a distinct 

 stomach, surrounded by a well-developed granular liver. The 

 intestine has a " neural flexure," and " either ends blindly in 

 the middle line, or else terminates in a distinct anus between 

 the pallial lobes." (Huxley.)- 



