STEtTCTrUE AND rHYSIOIOGY OF THE MOILUSCA. 39 



and consists of homy layers, sometimes hardened with shelly 

 matter (Fig. 28). 



It has been considered by Adanson, and more recently by 

 Br. Gray, as the equivalent of the dextral valve of the conchi- 

 fera; but however similar in appearance, its anatomical relations 

 are altogether different. In position it represents the hyssus of 

 the bivalves (Loven) ; and in function it is like the plug with 

 which unattached specimens of hysso-arca close their aperture. 

 — (Forbes). 



Homologies of the shell.* The shell is so simple a structure 

 that its modifications present few points for comparison ; but 

 even these are not wholly understood, or free from doubt. The 

 bivalve shell may be compared to the 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 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 both valves of the bivalve. In the ptcropoda it 

 consists of dorsal and ventral plates, comparable with the 

 valves of terebratida. In the gasteropoda it is equivalent to 

 both valves of the conchifera united above. f The nautilus shell 

 corresponds to that of the gasteropod ; but whilst its chambers 

 are shadowed forth in many spiral shells, the siphuncle is some- 

 thing additional ; and the entire shell of the cuttle-fish and 

 argonaut J have no known equivalent or parallel in the other 

 molluscous classes. The student might imagine a resemblance 

 in the shell of the orthoceras to a back-bone. The phragmocone 

 is the representative of the calcareous axis (or splanchno- skeleton) 

 of a coral, such as amplexus or siphonophyllia. 



Temperature and hybernation. Observations on the tempera^ 

 tii.re of the moUusca 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 ; 



* Parts ■which correspond in their real nature— (their origin and development) — 

 are termed homologous ; those which agree merely in appearance, or office, are said to 

 be analogous. 



t Compare fissurella or trochus (Fig. 28) with lepton squamosum (Fig. 12). The 

 disk of hipponyx is analogous to the ventral plate of hyalcea and terebratula, 



X The argonaut shell is compared by Mr. Adams to the nidamental capsules of tlie 

 tvhclk ; a better anal igue would have been found in the raft of the ianthina, which is 

 secreted by the foot of the animal, and serves to //uat the cgg-cai'sules. 



