48 MANUAL OF THE MOLLUSCA. 





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

 cut open and converted into separable valves. In the conchifera \ 

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

 and left. In the bracMopoda the separation is horizontal, and I 

 the valves are dorsal and ventral. The monomyarian bivalves lie ) 

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

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

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

 both 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 conchifera \ 

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

 gasteropod ; but whilst its chambers are shadowed forth in many ' 

 spiral shells, the siphimcle is something additional ;'and the entire i 

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

 or parallel in the other molluscous classes. The student might 'i 

 imagine a resemblance in the sheU of the orthoceras to a back-bone 

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

 neural and muscular cartilages of the cephalopod ; whilst its i 

 ])hra(jmocone is but the representative of the calcarious axis (or 

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



Temperature and hybernation. Observations on the tempera- 

 ture of the mollu8ca 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- 



* Qom^dxt fissurella or troclms (fig. 28) with Upton squamomvi (fig. 12). 

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

 bratula. 



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

 sules of the ivhelk ; a better analogue would have been found in the 7-aft of 

 the iantUna, which is secreted by the foot of the animal, and serves io float 

 the egg-capsules. 



