MOLLUSCOIDA. 175 



The inner surface of the valves of the 

 shell is lined by expansions of the integu- 

 ment, which are called the "mantle-lobes," 

 and which secrete the shell. The digestive 

 organs and muscles occupy a small space near 

 the apex or "beak" of the shell, which is 

 partitioned off by a membranous partition, 

 perforated by the aperture of the mouth. The 

 remainder of the cavity of the shell is almost 

 filled by two long processes, derived from the 

 sides of the mouth, fringed with lateral 

 branches, and termed the "arms." y These 

 arms are usually closely coiled up, and serve 

 to obtain food for the animal. It is from these 

 organs that the name of the class is derived 

 (Gr. bracMon, arm; and podes, feet). The 

 arms also serve as respiratory organs, and in 

 many forms they are supported on an internal 

 calcareous framework or skeleton, sometimes 

 called the " carriage-spring apparatus." 



The mouth is placed between the bases 

 of the arms, and is not furnished with any 

 apparatus of teeth. It conducts by a gullet 

 into a distinct stomach, surrounded by a well- FlG arJL"a, 

 developed granular liver. The intestine mav the muscular stalk 



j. i_ r i i -xi T j.- by which the shell 



or may not be furnished with a distinct anus, is attached. 

 but in no case does it open into the body- 

 cavity. Within the lobes of the mantle, there is a remarkable 

 system of branched tubes, which commence by blind extremi- 

 ties, and finally communicate with the mantle-cavity by means 

 of certain organs, which were formerly believed to be hearts, 

 and are now known as " pseudo-hearts." This system of tubes 

 appears to be mainly, if not entirely, connected with repro- 

 duction. A true heart, however, is present in most, if not in 

 all, of the JBrachiopoda. 



The nervous system consists of a single principal ganglion, 

 connected in some cases with others so as to form a collar 

 round the commencement of the gullet. In some cases, how- 

 ever, the nervous system appears to be very rudimentary. 



The sexes appear to be sometimes distinct and sometimes 

 united in the same individual. The embryo, in some cases, at 

 any rate, is locomotive, moving from place to place by means 

 of the ciliated arms or by ventral spines. 



