330 COMPAEATIVE ANATOMY. 



produce tlie varied forms of test and shell whicli characterise the 

 Molluscan phyhim. The hard structures^ therefore, of this division 

 differ essentially from those which are found in other classes of 

 animals, by the way in which they are developed. They are 

 products secreted from the body, and deposited on its exterior, and 

 are of great importance as organs for the support and defence of 

 the organisms to which they belong. They, just as much as other 

 differentiations from the integument, imply that the outer dermal 

 layer has a secretory activity. Although the outer layers of these 

 structures often appear to be — as is especially the case in large 

 shells — distinct from the organism, the shells always do form a part 

 of it, and are at many points directly and closely connected with 

 it; as, for example, at the insertion of the muscles into the shells. 



The presence of calcified spicules in the integument of the 

 Placophora calls to mind the relations which obtain in the Soleno- 

 gastres (p. 139). The spicules arise in follicles, and do not reach 

 the surface till they are larger in size, when they form slender and 

 closely-approximated fine processes, or thicker bodies, which are 

 distributed over the mantle. Eight large calcified plates are also 

 arranged transversely on it, and form a series of skeletal parts, the 

 arrangement of which implies that the body is arranged in a meta- 

 meric fashion. As in Cryptochiton they are covered by the mantle, 

 there is some reason for supposing that they were developed within 

 it, and that they resemble the spicules. The plates would then be 

 structures of the same kind, which had been greatly developed, 

 while the spicules would be parts which had not enlarged laterally, 

 but only vertically. This relation between the presence of a mantle 

 and the formation of firm organs, which, when largely developed, 

 form shells, is typical of all the other Mollusca ; and the two kinds 

 of organs are always intimately connected. Instead, however, of the 

 dorsal plates being developed, as in Chiton, the formation is con- 

 tinuous, so that it gives rise to a single shell. The shell, therefore, 

 just as much as the mantle — which we have seen to be homologous 

 throughout the series of Mollusca — must be regarded as an organ, 

 which is widely-distributed because inherited, great as may be the 

 adaptive modifications which it has undergone. 



The multifid shell was not replaced by the undivided one by any 

 new process, but by the development of one part, for we cannot 

 imagine that the shell, which, as an organ investing a large part of 

 the body, is one of so great functional importance, appeared all at 

 once. But, if the shell was at first an inconsiderable organ, it could 

 not have attained to that perfection of function which would have 

 been the cause of its being transmitted as a useful arrangement. 

 We must therefore suppose that the structure, which later on formed 

 the shell, was primitively one of several similar organs, and that it 

 gradually got the better of the rest. This gradual development of 

 the shell is the only mode which is intelligible, while at the same 

 time it connects together the multifid shell of the Placophora and 

 the single shell of the Conchifera. 



