STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 39 
and consists of horny layers, sometimes hardened with shelly 
matter (Fig. 28). 
It has been considered by Adanson, and more recently by 
Dr. 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 byssus of 
the bivalves (Loyén); and in function it is like the plug with 
which unattached specimens of bysso-arca close their aperture. 
—(Lorbes). 
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 conyerted 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 
he habitually on one side (like the plewronectide among fishes) ; 
and their shells, though really right and left, are termed 
“upper” and ‘‘lower” valyes. The univalve shell is the 
equivalent of both valves of the bivalve. In the pteropoda it 
consists of dorsal and yentral plates, comparable with the 
valves of terebratula. In the gasteropoda it is equivalent to 
both valves of the conchifera united above.t 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 { 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- 
ture of the mollusca are still wanted; it 1s known, however, to 
yary with the medium in which they liye, 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 /issurella or trochus (Fig. 28) with lepton squamosum (Fig. 12). The 
disk of hipponyz is analogous to the ventral plate of hyalea and terebratula. 
t The argonaut shell is compared by Mr, Adams to the nidamental capsules of the 
whelk ; a better analogue would have been found in the raft of the zanthina, which is 
secreted by the foot of the animal, and serves to //oat ihe egg-capsules. 
