x MOLLUSCA 339 



structure is now known as the velum. These additional cells remind 

 us of some of the " secondary trochoblasts " of Patella. 



The shell of the Trochophore is merely a thin horny cuticle 

 secreted by the cells of the everted shell gland. This cuticle adheres 

 closely to the ectoderm in the mid-dorsal line, and the ectoderm cells 

 here become columnar ; this region constitutes the hinge of the adult 

 shell. The cuticle adheres loosely towards the sides, but at the edges 

 of the shell gland a renewed deposition of cuticle takes place in two 

 small circular areas which rapidly extend in dimensions, and in this 

 way the valves of the bivalve shell are laid down. Not merely horny, 

 but also calcareous material is secreted by these shell-forming areas. 



coe 



a. 



FIG. 269. -Young Veliger larva of Dreissensia, polymorpha, seen from the side. 



(After Meisenheimer.) 



it, aims ; utlil.a, adductor muscle ; a.p, apical plate (afterwards becomes the anterior adductor muscle) ; 

 coe, rudiment of coelom ; c./i, cerebral pit ; tr.x, crystalline sac ; hep, lobes of liver ; in/, intestine ; l.-n, 

 larval kidney; m.ti; metutrocli ; <>, mouth ; p.it, pedal ganglion; r.d, dorsal retractor muscle; r.m, 

 middle retractor muscle ; /. <, ventral retractor muscle ; xli, shell; t.fr, telotrocli ; r.g, visceral ganglion ; 

 V, velum. 



The larva now changes its shape and instead of being cylindrical 

 becomes more or less laterally compressed. As the newly formed shell 

 valves extend towards the mid- ventral line, the mantle-cavity appears 

 as two longitudinal invaginations on the ventral surface (Fig. 268). 

 By the appearance of these grooves the edges of the area formed from 

 the everted shell gland are changed into right and left mantle-lobes. 

 The valves of the shell have a characteristic shape which appears to 

 be practically universal amongst the veliger larvae of Pelecypoda. 

 The hinge-line is straight and horizontal and the lower margin of 

 the valve is curved, so that the shape of the whole may be described as 

 semicircular (Figs. 269, 271). 



Behind the mouth, which is situated on a projecting oral cone, is 



