MOLLUSCA. Ill 



In Unio the valves are connected at the hinge-line by small projections, 

 teeth, which fit into corresponding sockets. 



In removing the animal from its shell the various muscles 

 attached to the impressions are cut through. They have corre- 

 sponding names. The adductor muscles (Fig. 32, V.S and H.S) 

 are broad bands of fibres which run transversely across from 

 one valve to the other, and by their contraction keep the shell 

 closed. 



Each shell is lined by a nap, the right or left lobe of the 

 mantle. These lobes have thickened edges which are attached 

 to the shell along the pallia! line by means of muscle-fibres 

 (Fig. 32, Mt). They are continuous dorsally with the wall of 

 the body, from which they grow out, are fused together above 

 and behind the posterior adductor (H.S), and immediately beyond 

 this are closely apposed to bound two oval openings. These are 

 a smaller exhalent or cloacal aperture (A) above, and a larger 

 inhalent aperture (E) below, the edges of which are fringed with 

 short tentacles. When at rest the mussel is completely imbedded 

 in the mud, with its posterior end projecting, so that water can 

 pass in at the inhalent and out at the exhalent aperture. In this 

 way the animal is supplied with food and oxygen on the one hand, 

 and gets rid of waste on the other. The mantle-lobes enclose 

 between them a large space, the mantle-cavity, which is divided 

 into two parts, a large branchial chamber below, and a small supra- 

 branchial chamber above. It must not be supposed that the 

 mantle lobes are fused posteriorly to bound the inhalent and 

 exhalent apertures, or ventrally to bound the branchial chamber. 

 If the animal is placed on its back the lobes can be separated 

 without cutting anything, as in Fig. 33, where by this means the 

 contents of the branchial chamber are exposed. In this figure 

 the smooth sides of the exhalent aperture have been separated, 

 and are seen just above A, on either side of which letter are the 

 fringed right and left sides of the inhalent aperture. A large 

 oval visceral mass (mesosoma), compressed from side to side, hangs 

 down into the branchial cavity, but there is no distinct head, nor 

 anything comparable to the lateral appendages of a crayfish. 

 The lower edge of the visceral mass is produced into a yellow 

 ploughshare-shaped muscular expansion, the foot' which projects 

 forwards (Fig. 32, F, and Fig. 33, P), and can be protruded from 

 the shell, serving as a locomotor organ. On either side of the 



