THE SHELLS OF MOLLUSCA. 335 



of fhe sliell, and forms tlie groundwork for tlie lamellar deposits, 

 wliicli often become very tliick on the inner face of the shell, the 

 carve of which is slight. These shells may be directly derived from 

 those of the Belemnites, especially if we take into consideration those 

 shells which have a free projecting tip, like the shell of S. Orbig- 

 niana. The solid tip corresponds to the rostrum of the Belemnites, 

 while the alveolar cavities of these latter, and the pro-ostracum, which 

 is continued on from their dorsal surface, is homologous with the rest 

 of the shell of the Sepiadte. The partitions which form the chambers 

 of the phragmocone in the alveoli of the Belemnites are represented 

 by the flat or slightly concave lamellae of the shell of the Sepiadte. 

 The layers succeed one another directly, instead of forming separate 

 chambers. In this way the complicated shell of the Belemnites, 

 when reduced, may be easily seen to be represented in a lower con- 

 dition of the shells of the Sepiada3. The shell of the Loliginidfe is 

 still more reduced; it is merely formed by an elongated flexible 

 horny-blade (calamus), which is placed in the dorsal region of the 

 mantle. An outwardly projecting carina extends along the middle 

 line. This rudimentary shell corresponds to the external, curved, and 

 more highly organic portion of the shell of the Sepiadae, and is there- 

 fore homologous with the horny-blade of a Belemnite shell. Finally, 

 in the genus Octopus, where the mantle is not separated from the head 

 in the region of the neck, we find a pair of thin plates, embedded in 

 the dorsal integument ; these are the last traces of a shell formed 

 by the mantle, and are in all respects comparable to those described 

 as existing in the Gastropoda. Inasmuch as the shell is, even in the 

 Cephalopoda, formed in the earliest stage by an invagination of the 

 mantle (Sepia), the internal and external shells are closely allied ; and 

 at the same time we may see that they are connected with the shells 

 of other MoUusca. v 



The shell of Argonauta is to be regarded as altogether different 

 from all these shells, which are intelligible when closely compared; it is 

 not secreted by the mantle, but by a pair of arms, which do not layj 

 down lamellar deposits. In the Gastropoda we met with a special 

 arrangement by which the so-called " operculum " was formed ; we 

 found this on the dorsal surface of the end of the foot in many 

 Prosobranchiata, where it served to shut in the animal, when it was 

 retracted into its shell. The question now arises, may not this 

 structure be also derived from one plate of the Placophora ? 



[E. Eat La.nkestee, Developt. of the Pond Snail. Quarterly Journ. Microsc. Sci. 

 1874.] 



Branchiae. 



§ 260. 



The kind of respiratory organs — ^branchife — which obtains in the 

 Mollusca is in correlation with their aquatic habitat ; these are always 

 differentiations of the integument, and have consequently a 



