386 THE SHELL OF MOLLUSCA : 



remember that the central foot, the principal organ of pro- 

 gression in the Mollusca, has either no connection with the 

 shell or an indirect and remote one. Hence we find that 

 when the shell is too small to contain and shelter the entire 

 body it is placed over the heart and respiratory organs, to 

 protect from hurt those, beyond question, most essential to 

 the animal's existence. 



Such a shell, however, is not the exclusive possession of 

 the Mollusca. I do not mean to compare with it the tes- 

 taceous covering of the acorn-shells or of the barnacles 

 (Cirrhopoda), for the differences between them in their com- 

 position or combination of parts are so great as to render 

 their distinction easy ; but there is a genus of crustaceous 

 insects (Cypris) whose covering very exactly resembles the 

 shell of a minute bivalved Mollusk ;* and without a know- 

 ledge of the inhabitants it must be difficult, if not impracti- 

 cable, to distinguish the calcareous case of some marine 

 worms from the tubular shells of certain Gasteropods. The 

 sameness is so near that the best naturalists misarranged the 

 latter until the animals of them were discovered ; and I may 

 just remind you that the microscopical chambered shells, long 

 believed to be the productions of Cephalopods of commensu- 

 rate diminutiveness, are now more justly considered to be- 

 long some of them to the Annelides, and others of them to 

 much inferior organisms. -j- To establish a line of demarca- 

 tion between these productions and the shells of the Mol- 

 lusca is, perhaps, important in a geological view, but I am 

 not aware of any that can be in every case made apparent. 

 The points in the interior of the shell on which the muscles 

 of the Mollusca are inserted receive a mark or impression 

 from them which is ever afterwards retained, and when such 

 marks are observed, the conclusion of the shell being a Mol- 

 lusk's is very certain, for no Annelide is fixed within its 

 tube, but resides within at liberty to leave it on an emer- 

 gency, and hence no constant marks on the parietes are ever 

 made. But these marks, even when existing, may be too 

 faint to be always perceived, or the smallness of the shell 

 may present an insurmountable barrier to the search of the 

 test ; and in this dilemma I know of no other that you can 

 unhesitatingly depend upon. 



* " The great affinity tliat the coverings, or sliells, of some of tliis class 

 of insects bear to the tcstacea tribe, has in all probability caused many to 

 have been considered as small species of Mytilus, or the fry of larger ; for 

 many such Monocnli are capable of shutting their valves entirely, and in- 

 closing every part of the animal ; in which state they always are when dead, 

 so that it is no easy matter to discriminate." — Montagu Test. Brit. 174. 



f Dujardin, Hist. Nat. des Infusories, p. 240 — 45. 



