THE CONCHOLOGIST, 



change of shell takes place, the embryonic 

 shell being cast, a new shell being formed 

 in its place. The typical larva of a mollusc, 

 as Lankester has pointed out, is essentially 

 similar to the larva of a number of inverte- 

 brate types, and especially the Chtetopoda, 

 with the addition of certain special organs 

 characteristic of the moUusca. The charac- 

 teristic molluscan organs are : —a foot between 

 the mouth and anus, and an invagination 

 of the epiblast of the dorsal side at the 

 hinder end of the body, which is connected 

 with the formation of the shell, called the 

 shell-gland. Within this shell-gland the larval 

 shell is more commonly preceded in its 

 development by the appearance of a plug 

 of chitinous matter. There is a definite 

 order in the embryonic development of the 

 molluscan larva up to this point, but in its 

 subsequent development and metamorphosis 

 the subject becomes complicated, proving the 

 existence of original disturbing conditions. 

 These have produced corresponding effects 

 upon the animal, and consequently upon the 

 form, and in some measure the production 

 of an external shell. These disturbing 

 elements may have been due to the differ- 

 ences induced in their surrounding conditions, 

 or to causes of which we can learn nothing 

 from their embryological history. That some 

 active influences, even in the waters of the 

 sea, have been present, is shown by the fact 

 that many of the molluscan larva when 

 hatched have an embryonic shell, others have 

 not ; and in the case of some Pteropoda, 

 the larva possesses an embryonic shell, a 

 secondary shell being added during larval 

 life. While in the free-swimming larval con- 

 dition the animal would be very easily 

 influenced by both animate and inanimate 

 surroundings. 



Having now briefly reviewed the physio- 

 logical and embryological characters of the 

 molluscan shell, what do we learn from them 



with regard to the question as to their 

 origin. It has been remarked that the first 

 appearance of the embryological shell is 

 preceded by a chitinous plug, and that in 

 the formation of the shell proper of the adult 

 animal, the chitinous epidermis or perios- 

 tracum is first formed. As the oldest or 

 earliest developmental animal characters are 

 the first to make their appearance, it may 

 reasonably be argued that the earliest cover- 

 ing, protective or otherwise, was a simple 

 external chitinous covering, which has during 

 a long period of time become modified into 

 the varied forms belonging to the moUusca. 



The next character in order of formation 

 in the adult shell is the prismatic layer, which 

 has been shown to be the crystalline form 

 of aragonite, or carbonate of lime. This 

 may originally have been formed by the 

 secretion of more or less numerous small 

 crystals of carbonate of lime — as in some 

 modern Tunicates, which have scattered 

 through their outer covering small spicule 

 of carbonate of lime — which, by becoming 

 more numerous, would form a thin prismatic 

 layer. This deposition of crystals may have 

 been caused in the first instances by an 

 undue amount of the calcic carbonate in 

 their food, which could not be properly 

 excreted or carried away by the usual 

 methods, consequently would be circulated 

 through the body, and become secreted on 

 the outer parts of the animal. 



As the small crystals would be unable to 

 pass through the original chitinous epidermis, 

 they would thus naturally aggregate together 

 upon its inner side. The nacreous layer 

 being the last formed, is probably a later 

 development of the secretive power of the 

 animal. 



Upon this view, the periostracum is the 

 original covering, and the carbonate of lime 

 shell a secretory product made use of by 

 the animal, a service that has been trans- 



