ON THE DEVELOPMENT OF LINGULA ANATINA. 47 



arm-sinus, but later five more diverticula of the body cavity are 

 developed ; viz : two pairs of the palliai sinuses, and an unpaired 

 peduncular cavity. Here I will confine myself to the description 

 of such parts of the wall of the body cavity as have no definite 

 relations with such organs as the ganglia, otocysts, nephridia, 

 palliai sinuses, or peduncle. The body wall is composed of two 

 layers, as already stated, the inner being the parietal layer of the 

 mesoblast, and the outer the ectoblast. 



The lateral body wall remains in its original state in the 

 larvoe of the o p. c. stage, but in the p. c. stage the cells of 

 the peritoneum (parietal layer of the mesoblast) proliferate special 

 cells outward just beneath the epidermis. From these cells are 

 formed the muscle fibres which now come to constitute the 

 parietal muscles (PI. VI., Fig. 85., and PL VII., Figs. 93, 94., 

 7)i. pr.). These fibres run longitudinally along the lateral body 

 wall, bending toward the median plane near the hinge region. 

 Even at the 8 p. c. stage the muscle layer remains very thin, 

 being but a few fibres thick. The ectoderm is extremely flattened 

 throughout. 



On the dorsal and ventral faces the body wall is for the 

 most part so exceedingly thin that it becomes very difficult to 

 make out the two layers of which it is composed. 



To describe first the outer layer. Notwithstanding its thin- 

 ness, it maintains the power of secreting shell substance throughout 

 life, to which alone the increase in thickness of the shell is 

 attributable. At the insertions of muscles this ectoblastic layer 

 is modified into muscular tissue (Haftzellen, Blochmann), a 

 character which will be referred to more fully in the section on 

 the muscular system. 



The inner layer, or peritoneum, is equally thin. This layer 



