42 COMPARATIVE ANATOMY. 



as in the Lamellibranchiata, but dorsal and ventral in relation 

 to the animal they contain. Between the margins of the 

 umbones the long muscular peduncle by which the animal 

 is attached is protruded. The dorsal valve and mantle of the 

 right-hand specimen have been removed, but the ventral mantle- 

 lobe, with its margin and fringe of setse, is left intact, adhering 

 to the inner surface of the ventral valve. Little of the body, 

 which occupies but a small part of the shell, can be distinguished, 

 with the exception of a white glandular central mass, the liver, 

 and the various muscles by which the adduction and adjustment 

 of the valves is effected. The lateral margins of the mouth 

 are produced outwards into two muscular ciliated ' arms/ which 

 occupy quite one-half of the shell, and are curved inwards to- 

 wards each other. ,The bases of the arms are connected with 

 each other by a vertical membranous partition. 



Division II. MOLLUSCA PKOPER. 



Class LAMELLIBRANCHIATA. 



66. A Fresh-water Mussel (Anodonta cygnea), removed from 

 its shell, so as to shew the general relation of the mantle and 

 branchiae. The two strings by which the animal is suspended 

 are attached to the mantle, one to each of its lobes. Bristles 

 are inserted into the mouth and anus. 



The mantle consists of two lateral lobes, confluent with each 

 other along the medio-dorsal line, but free throughout the whole 

 of their ventral margins. With the exception of the free, thick- 

 ened, glandular, and shell-secreting ventral margin, which is 

 homologous with the ' collar ' of Gasteropoda, each pallial lobe is 

 comparatively thin, and in the fresh condition nearly trans- 

 parent. In the vicinity of the anus the free edge of each 

 mantle-lobe is beset with a fringe of minute papillae. This 

 papillose border corresponds with that part of the mantle which 

 in the siphonate Lamellibranchiata is produced into the cylin- 

 drical exhalent and inhalent siphons. There are two pairs of 

 branchiae on each side of the central body-mass ; each gill is a 

 flattened bag composed of longitudinal and transverse fibres, 



