THE SHELL OF MOLLUSCA. 331 



§ 258. 



The earliest rudiment of tlie shell appears at the aboral end of 

 the embryo, at a spot which is distinguished by the growth of its 

 ectoderm. A viscid substance is secreted in the gland-like invagi- 

 nation which appears at this point (Fig. 174, s). This substance 

 gradually fills up the invagination and 

 reaches the surface, where it is hardened 

 as soon as it comes in contact with the 

 water (s). When the invagination dis- 

 appears its edges remain as a raised ridge, 

 and so form the rudiment of the mantle, 

 which is therefore very closely connected 

 with the formation of the shell. This ar- 

 rangement, which has been made out in 

 the larger divisions of the Conchifera, 

 points to the common origin of this group, pig. 174. Embryo of a Hete- 

 while it also affords an explanation of the ropod, transverse section. 

 cause of the different ways in which their " ^?J'^^- l^f'^'^-J ^^^'^'^^^ 



-in n ^ ^Tr1 j.i • • • cavity, i? Foot, s Shell-gland 



shells are formed. When this mvagma- (after H. Tol). 



tion disappears the shell becomes external, 



and then the edge of the mantle either remains below it or more or 

 less covers it. The latter arrangement shows how the external shells 

 are connected with the internal ones, which are formed when the 

 invagination does not disappeai', but becomes still further developed 

 in the manner already indicated. The shell is then secreted from the 

 inner face of the walls of this organ, and it varies greatly in character 

 in various divisions, just as do the external shells. 



When simplest, all the lamellae of the shell are similar in 

 character ; in many, when in their lowest conditions, it is perforated 

 by pore-canals. The simple condition is complicated by the 

 appearance of layers of prisms set obliquely or perpendicularly to 

 the lamellae. 



The shell increases in surface at its free edge, the deposits 

 occurring in layers, and at the side of the mantle ; superficially they 

 have the appearance of concentric rings. The shell becomes thicker 

 internally by being supplied from the outer surface of the mantle. 

 These varying modes of formation give rise to variations in the 

 structure of the formed shell, the inner portion of which often 

 consists of a large number of superjacent folded layers, the presence 

 of which is the cause of the nacreous appearance of the shell (mother- 

 of-pearl). These layers are covered by the external more compli- 

 cated and compound layers, which are formed by the edge of the 

 mantle. The horny covering (periostracum) of many shells is due 

 to the same part. 



The shell of the Lamellibranchiata, like the mantle, is developed 

 on either side of the body, but it is not calcified in the middle line, 

 so that it forms two valves, which are connected with one another 



