STEUCTTJKE AND PHYSIOLOGY OF THE MOLLUSCA. 39 



and consists of horny layers, sometimes hardened "witii shelly 

 matter (Fig. 28). 



It has been considered by Adanson, and more recently by 

 Dr. Gray, as the equivalent of the dextral yalve of the conchi- 

 fcra; butho'^eyer similar in appearance, its anatomical relations 

 are altogether different. In jDosition it represents the hjssus of 

 the biyalyes (Loyen) ; and in function it is like the plug with 

 Avhich unattached specimens of hysso-arca close their aperture. 

 — [Forles). 



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

 that its modifications present few jioints for comparison ; but 

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

 Invalve shell maybe compared to the outer tunic of the ascidian, 

 cut oi^en and conyerted into separable yalyes. In the conchifera 

 this division of the mantle is yertical, and the yalyes are right 

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

 the yalyes are dorsal and yentral. The monomyarian biyalyes 

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

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

 "upper" and "lower" yalyes. The uniyalye shell is the 

 equivalent of hoth yalyes of the biyalye. In the ^teropoda it 

 consists of dorsal and yentral plates, comparable with the 

 yalyes of terehratula . In the gasteropoda it is equivalent to 

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

 corresponds to that of the gasterojDod ; but whilst its chambers 

 are shadowed forth in many S25iral shells, the siphuncle is some- 

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

 argonaut \ 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 hack-hone. The phragmocone 

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

 of a coral, such as amplexus or sipihonophylUa. 



Temperature and hyhernation. Observations on the tempera- 

 ture of the mollusca 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 whicli correspond in their real natui'e — (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. 2S) \N-ith lepton squamosum (Fig. 12). Ilie 

 disk of Jiipponyx is analogous to the ventral plate of hyal<ca and terebratula. 



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

 iihclk ; a better analogue would have been found in the raft of the ianthina, whicli is 

 secreted by the foot of the animal, and serves to /Innt the egg-capsules. 



