306 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



arrangement, or disposition of the layers of the shell 

 in horizontal planes, is well seen in the oyster. Where 

 the possessor of the shell is in the habit of floating 

 (lanthina, Argonauta), the "shell" is comparatively 

 thin, and of Jow specific gravity. 



In the most primitive Mollusca the shell is merely 

 spicular ; in Neomenia the spicula are arranged in a 

 single layer in the outer part of the integument, and 

 protrude their pointed ends from its surface ; in 

 Proneomenia the spicules are set in several layers, 

 placed in a chitinous interspicular substance. In both 

 cases the spicules consist of carbonate of lime. A 

 higher stage is found in the Chitons, where the shell 

 consists of eight plates which lie on the back of the 

 animal, and have an arrangement which we can 

 hardly resist from regarding as metameric. In the 

 Lamellibrancliiata the shell consists of two 

 valves, which lie to the right and left of the animal, 

 and are connected with one another by a chitinoiis 

 ligament, which runs along the middle line of the back; 

 this ligament is elastic, and in consequence of this pro- 

 perty, the animal in a state of repose is able to keep 

 its two valves slightly separated without incurring any 

 expense of muscular activity. At the approach of 

 danger the valves can be approximated so as to pro- 

 tect their possessor, by the contraction of two (fresh- 

 water mussel) or one (oyster) pair of adductor muscles. 

 In the higher Cephalophora the shell is always 

 single, whence they are often distinguished by the name 

 of univalves from the Lamellibranchs, which are the 

 bivalves. This shell may form a single conical cup, 

 as in the limpet (Patella), or it may be slightly coiled, 

 as in the cowry, or it may be greatly coiled and 

 consist of a number of chambers, as in the nautilus. 

 It is certain that many conch ologists go too far in the 

 trust that they put in the characters of the shell, but 

 it remains as a matter of fact that in the creat 



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