62 



COMPARATIVE ANATOMY 



CHAP. 



The Lamellibranch shell is originally symmetrical, that is to say, the two valves, 

 apart from the almost invariable asymmetry of the hinge, are exactly alike 

 (equivalve). This is the case in most of the Lamellibranchia. The two valves 

 may, however, become unlike, i.e. the shell (and to a much lesser extent, and only 

 in unimportant details, the soft body also) may become asymmetrical. As far as 

 we can at present judge, this asymmetry is caused by adaptation to an attached 

 manner of life. 



The left valve of the Oyster is firmly cemented to the surface on which it 

 rests. This valve is thicker, more convex and spacious, and forms a sort of 

 basin in which the soft body lies, while the right valve acts rather as a lid, and 

 is thinner and flatter. We have thus an 'upper" (the right) and a "lower" (the 

 left) valve, but it is hardly necessary to point out that this use of the terms upper 

 and lower has as little morphological significance as in the Pleuronectidae among the 

 fishes. The attached valve is sometimes the right, sometimes the left, and this 



Fio. "i. Diagram in illustration of the mechanism for opening and closing the Lamelli- 

 branch shell. 1, 2. 3, The three layers of the shell 1, prismatic layer ; 2, cuticle or periostracum ; 

 3, nacreous layer. A, Shell closed by the contraction of the adductor muscle (6), by means of 

 which the elastic inner portion of the ligament (5) is compressed. B, Shell opened by the elastic 

 pressure of the inner portion of the ligament during the relaxation of the adductor muscle. 4, 

 Xon-elastic outer portion of the ligament, which passes into the periostracum. 



variation may occur within one and the same genus (Chama), or even species 

 (Aetheria). 



Besides the above-named, the following bivalves are also attached, and have 

 dissimilar valves : Spondylus, Gryphcea p. p., Exogyra p. p., and especially the 

 fossil Hippuritcs (Rudistes), in which the right valve assumes the form of a high cone 

 attached by its point, while the left looks like a lid. The conical valve has, however, 

 no corresponding internal cavity, but is almost entirely filled up with shell substance, 

 so that, in spite of the form of the shell, the space occupied by the animal between 

 the two valves is very limited. 



This same condition is found in certain fossil Chamacca. In Requienia, the left 

 valve is produced spirally and is attached by its point, while the spirally-coiled 

 flattened right valve covers it like a lid, so that the whole shell closely resembles 

 a Gastropod shell closed by its operculum. 



There are also free, unattached bivalves with unequal valves, e.g. many 

 Pectinidce. In these animals, however, many peculiarities of organisation, such as 



