SEA MUSSEL MYTILUS EDULi;. 1 95 



left sides of the body, where they may be seen lateral to the stomach and inside the 

 prodissoconch at the stage when the larva has attained a length of 0.274 mm - ( n g- I %4> 

 Dis, p. 197). The dissoconch continues to grow rapidly in size by further deposition 

 of calcareous matter. By the time the young mussel has reached a length of 0.512 

 mm. the prismatic shell can be seen extending far beyond the limits of the pro- 

 dissoconch (fig. 187, Dis, p. 198). At this stage the form of the developing mussel 

 is undergoing a radical change from the more or less circular, straight hinge-line embry- 

 onic shell to the triangular ovate form of the adult (figs. 187 and 188, p. 198). This is 

 accomplished by growth taking place most rapidly in the ventral and posterior directions. 

 The prodissoconch persists as the covering, periostracum, of the umbones and is shown 

 in its final position in the 0.72 mm. stage (fig. 188, Pds). 



ALIMENTARY ORGANS. 



In the trochophore larva it has been shown that the oesophagus is large and leads 

 from the posterior border of the velum to the stomach. The intestine is short and 

 straight, leading directly from the posterior end of the stomach to the anus, which, at 

 this stage, is located a short distance behind the oral opening. During the transition 

 from the larva to the adult the intestine increases in length, which results, first, in a 

 bending to the left with the formation of a loop. The portion anterior to the loop grows 

 directly backward to form the direct intestine, while the loop continues to lengthen in 

 the anterior direction on the left side of the stomach until it reaches the oesophagus. 

 This results in the formation of the recurrent and the terminal portions of the intestine 

 (fig. 188, RI and 77, p. 198). 



When the larva is about 0.27 mm. long the liver appears as a pair of diverticula 

 composed of large, loosely aggregated endodermal cells from the anterior lateral walls of 

 the stomach. In a short time they become tinged with a brownish pigment, which is 

 characteristic of the gland and makes it stand out distinctly from the other tissues, 

 (fig. 1S4, L, p. 197). By the time the embryo has attained a length of 0.72 mm. the 

 liver tissue has grown to envelop the stomach completely (fig. 188, St). 



The labial palps, to all appearances in total preparations, are developed the same 

 way in Mytilus as Meisenheimer (1901) observed them in Dreissensia polymorpha. They 

 arise from the cerebral pit, after the fundaments of the cerebral ganglia have been laid 

 down, by a flattening of the tissue into two lateral bands of ciliated epithelium above 

 and at the sides of the mouth (fig. i87,LP,p. 198). From these, the superior and inferior 

 palps are developed by growth from the upper and lower edges, respectively. 



MUSCLES. 



As the end of the larval period approaches the various systems of muscles, charac- 

 teristic of the adult, develop in rapid succession. According to Wilson (1887) the pallial 

 muscles appear first as a band of considerable width running round the entire margin 

 of the valves before the embryo is 0.134 mm. long, or about 12 days old. Then the an- 

 terior adductor muscle is formed from a group of large mesoblast cells which appear 

 in the anterior region (fig. 184, AAd, p. 197). This is followed immediately by the 

 development of the posterior adductor muscle from a similar group of mesoblast cells 

 in the posterior dorsal region (fig. 184, PAd). At this stage the anterior adductor is 



