60 



LAMELLIBRANCHIA. 



6. The Formation of the Organs.* 

 A. The Shell. 



The shell, as in the Gastropoda, is unpaired in its origin, and is 

 formed by a secretion of the epithelium of the shell-gland (Figs. 14, 

 p. 28, and 15, p. 31). This embryonic cuticular shell is retained 

 and passes over into the periostracum (epidermis) of the adult shell. 

 The latter arises through a secretion of granular calcareous substance ; 

 this at first accumulates in two complexes lying symmetrically at the 

 two sides of the body beneath the cuticular shell (Fig. 21 A, p. 44), 

 which, by further increase, yield the shell-valves. These grow out 

 dorsally until they meet. The dorsal part of the cuticular shell that 

 lies between the two calcified shell-valves which are growing towards 

 one another yields the ligament of the shell (ZIEGLER, p. 43). 

 The larval shell which thus arises and which has a very simple 

 structure, is retained in Ostrea, the Unionidae and, as has recently 

 been proved, in a number of other Lamellibranchs (KYDER, JACKSON, 

 BRAUN, SCHIERHOLZ). It is found as a minute prominence on the 

 umbo of the large shell when the latter has not, as is often the case 



with the Unionidae, been de- 

 ft, stroyed through mechanical 



The larval shell usually differs in 

 shape from the shell of the adult, 

 and is even sometimes very unlike 

 the latter. As a rule, the shell 

 changes very much during develop- 

 ment. The youngest stage of the 

 bivalve shell is characterised by a 

 straight hinge-line and the slight 

 development of the umbones. This 

 can be seen in Fig. 27 A, and still 

 better in Figs. 14-16, pp. 28-33). This 

 stage (Fig. 15 C, p. 31), follows 

 directly upon that of the unpaired 

 cuticular shell, and must be supposed 

 to have arisen out of the latter by the 

 gradual swelling and upward growth 

 of the shell on either side of the dorsal 



middle line, causing a slight infolding along this line which gives origin to 

 the straight hinge. This straight hinge changes, as the shell soon begins to 



FIG. 27. A and B, early stages of the 

 larval shell of Ostrea edulis, seen from 

 the side and from behind ; O, somewhat 

 later stage (" prodissoconch ") of the 

 larval shell of Ostrea virginiana, oblique 

 side view. This particular larva had 

 just attached itself, the left shell being 

 already lightly fastened to the sub- 

 stratum (after JACKSON). 



* We here treat of the formation of the organs only in so far as this has not 

 already been considered. 



