48 MANUAL OF THE MOLLUSCA. 



bivalve shell may be compared to the outer tunic of the ascidian, 

 cut open and converted into separable valves. In the concJiifera 

 this division of the mantle is vertical, and the valves are right 

 and left. In the brachiopoda the separation is horizontal, and 

 the valves are dorsal and ventral. The monomyarian bivalves lie 

 habitually on one side (like the pleuronectidce among fishes) ; and 

 their shells, though really right and left, are termed " upper" 

 and " lower" valves. The univalve shell is the equivalent of 

 loth valves of the bivalve. In the pteropoda it consists of dorsal 

 and ventral plates, comparable with the valves of terebratula. In 

 the gasteropoda it is equivalent to both valves of the concJiifera 

 united above.* The nautilus shell corresponds to that of the 

 gasteropod ; biit whilst its chambers are shadowed forth in many 

 spiral shells, the siphuncle is something additional ; r and the entire 

 shell of the cuttle-fish and argonautf have no known equivalent 

 or parallel in the other molluscous classes. The student might 

 imagine a resemblance in the shell of the ortkoceras to a lack-bone ; 

 but the true homologue of the vertebrate skeleton is found in the 

 neural and muscular cartilages of the cephalopod; whilst its 

 phragmocone is but the representative of the calcarious axis (or 

 splanchno-skeleton) of a coral, such as amplexus or siphonophyllia. 



Temperature and hybernation. Observations on the tempera- 

 ture of the mollmca are still wanted ; it is known, however, to 

 vary with the medium in which they live, and to be sometimes a 

 degree or two higher or lower than the external temperature; with 

 snails (in cool weather), it is generally a degree or two higher. 



The mollusca of temperate and cold climates are subject to 

 hybernation ; during which state the heart ceases to beat, respira- 



* Compare fssurella or trochus (fig. 28) with lepton squamosum (fig. 12). 

 The disk of hipponyx is analogous to the ventral plate of hyalsea and tere- 

 bratula. 



f The argonaut shell is compared by Mr. Adams to the nidamental cap- 

 sules of the whelk ; a better analogue would have been found in the raft of 

 the ianthina, which is secreted by the foot of the animal, and serves iv float 

 the egg -capsules. 



