THE DEVELOPMENT OF LAMELLIBRANCHS, 



331 



of the egg-shell, in Fig. 175. The two valves of the shell 

 are united by a hinge, and they are somewhat triangular 

 in side view. The elasticity of the hinge ligament is so 

 great that it may open the valves 

 until they lie in the same plane. 

 The ventral angle of each valve 

 is bent inwards to form a mova- 

 ble toothed hook (Figs. 174 and 

 175, A), from which the larva 

 has received its name " Glochi- 

 dium." 



FIG. 175. Ventral view of the same 

 larva, with the valves of the shell opened. 

 (Drawn from nature by W. K. Brooks.) 



Letters as in Fig. 174. 



The valves are lined by the large spherical cells of the 

 mantle, and from some of them large stout setae (s) pro- 

 ject into the mantle cavity. The valves are closed by a 

 very large and well-developed adductor muscle (m) ; but 

 the elasticity of the hinge ligament is so great that re- 

 peated efforts are necessary before the animal can close the 

 shell completely after it has been thrown open. The space 

 between the halves of the mantle is usually almost entirely 

 filled by a long, clastic, tough, brown, coiled thread, the 

 /jy.w.s', which is shown at b. The byssus is formed in a 

 long, tubular byssus organ (bg) which is coiled inside the 

 left valve of the shell, between it and the cells of the 

 mantle. The Glochidium has no ears or eyes, no gills 

 and there is no projecting locomotor velum, although 

 a row of cilia (?), at the anterior end of the body, 

 may be a rudimentary velum. The digestive cavity 

 is not divided into regions, but is a simple pouch with 



