34 LAMELLIBRANCHIA. 



larvae of the last-named form are met with swimming freely on the surface of 

 the water before attaining the TrochopJwre stage as well as at that stage. 



Before indicating further points of resemblance between the 

 Lamellibranch larvae and the Anne-lidan Trodiophore connected speci- 

 ally with the internal organisation, we must first draw attention to 

 a character, not hitherto considered, which distinguishes these larvae 

 at once from all other (non-Molluscan) larvae. This is the so-called 

 shell-gland. At a somewhat early period in O-strea, as early as the 

 ^■astrula-stage (Fig. 14 B), in Teredo rather later, a part of the 

 ectoderm, which is somewhat thickened by the lengthening of its 

 cells, forms a trough-like depression on the dorsal surface near the 

 posterior pole (Fig. 15 B). This depression, which represents the 

 rudiment of the shell-gland, soon deepens considerably, so that it 

 appears like a blind tube (Figs. 14 G, and 22, p. 50), It has a 

 glandular character, inasmuch as its cells show the longitudinal 

 striation characteristic of many glandular cells : it soon also begins 

 to secrete a substance which can be seen as a thin integument over 

 the external aperture and the margin of the shell-gland (Figs. 14 C, 

 and 15 B). This is the first indication of the shell, and it is thus 

 seen that the latter in its earliest rudiment is unpaired. 



In the further course of development the invagination of the 

 shell-gland flattens out again, first becoming induced to a shallow 

 depression covered by the rudiment of the shell (Figs. 14 D, and 23), 

 and later disappearing altogether. The shell at the same time in- 

 creases in size, and now, like a saddle, covers a part of the dorsal 

 and lateral surfaces (Figs. 15 C, 14 E, and 23 C). By the extension 

 of the shell over the sides of the larvae, the way is prepared for the 

 duplication of the former, and very soon a median dorsal dividing line 

 can be seen separating the shell into two laterally situated valves. 

 This line corresponds to the hinge-margin of the adult shell ; it is indi- 

 cated in Figs. 15 C and 14 A' by the straight line on the back of the 

 larva (';/'. also the method of formation of the definitive shell in 

 ( 'ycla*, p. 43). The large size subsequently attained by the shell in 

 the free-swimming larva is to be seen in Figs. 1 (1 and 17 Z? and C. 

 Tlir shell is seen to project beyond the body, a condition only ren- 

 dered possible by the formation of the right and left mantle-folds 

 which has already taken place. These folds are formed as lateral out- 

 growths of the ectoderm, the outer layer of ectodermal cells being in 

 close contact with the shell, while the inner surface of the outgrowth 

 is separated from the keel-shaped ventral region of the larva by a 

 deep fissure — the mantle-cavity (Hatschek). The reader should 



