BRACHIOPODA. 



2O I 



Fig. 141. Minute structure 

 of the shell of Terebratula, 

 showing the flattened prisms 

 of the shell, and the canals. 



composed of one or two pieces, and which is termed the " del- 

 tidium" (fig. 140, m). In other cases this structure is alto- 

 gether wanting. 



In intimate structure, the shell of most of the Brachiopoda 

 (fig. 141) consists " of flattened prisms, of considerable length, 

 arranged parallel to one another with great regularity, and at a 

 very 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 "punc- 

 tated " structure, and in the living ani- 

 mal they contain csecal 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 Rhynchonellidce) 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. 138, 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." 



In some forms, as in the typical Terebratulce (fig. 138, 2), the 

 internal skeleton which supports the arms is a short shelly loop, 

 of a very simple character. In these cases it is only at their 

 bases that the arms are supported, and they are therefore 

 more or less movable. In other cases, as in the Spiriferidcz 

 (fig. 142), the arms must have been immovable, as they are 

 supported by two thin spirally-foiled lamellae, which form two 



