OR OSSEOUS SYSTEM. 45 



anterior part of the shell. The muscular foot (f } ) extends 

 from the ventral surface of the body, where we see also 

 the byssus (g,) by which the delicate mytiloid shells are 

 fixed to solid bodies. The respiratory orifice (^,) is placed 

 at the most open part of the shell, and near this is the 

 vent (i,) by which the respired water and the excretions 

 are discharged. The rectum (/<:,) opens near to the vent 

 after perforating the ventricle of the heart. The large lobes 

 of the liver (/,) envelope the turns of the intestine, and the 

 pyloric portion of the stomach . On the ventral or inferior 

 surface of the abdomen, is the respiratory cavity in which 

 are seen the four long pectinated laminae of the gills (m,) 

 and towards the posterior part of the shell is the strong 

 adductor muscle (n,) by which the valves are drawn forcibly 

 together when the animal is alarmed. The ovary (o,) 

 fills a great portion of the posterior and upper part of 

 the shell. From this position of the soft parts within the 

 valves, we perceive the anterior part (d 9 ) of the shell to 

 be that next the mouth (e,) and the posterior part (b,) 

 to be that next to the anus (&,) and the vent. The 

 dorsal margin (a,) of the valves is that next to the liver 

 and ovary, and the ventral part is that which encloses 

 the respiratory gills (m) ; we also observe from these 

 parts the reason for designating the one valve (,) the 

 right, and the other (b,) the left, as we term the lateral 

 parts of the skeleton in higher animals. As these shells 

 of conchifera are extravascular exudations from the surface 

 of the mantle, they are incapable of growth but by su- 

 perposition of new parts, and the various spines and 

 lamince, which are so often developed on their exterior 

 surface are mere depositions from the edge of the mantle, 

 and indicate former positions of the margin of the valves. 

 The ordinary direction in which these processes of the 

 shells are formed is seen in the long narrow projecting 

 laminae of the spondylus gcederopus, (Fig. 21. 1 .) where 

 they are arranged, with great symmetry, in rows which 

 are regular both in the longitudinal and transverse direc- 

 tions, and indicate the positions of the processes of the 

 mantle which excreted them. The adductor muscle of the 

 valves leaves a depression on the inner surface of each, 

 which marks its place of insertion and its form, and which 



