MOLLUSCA; GENERAL CHARACTERS. 369 



shell. When there is either no shell at all, or merely a rudi- 

 mentary shell enclosed in the mantle, the Mollusc is said to 

 be " naked." The shell of the " testaceous " Mollusca is very 

 closely related to the respiratory organs ; " indeed it may be 

 regarded as a pneumoskeleton. being essentially a calcined por- 

 tion of the mantle, of which the breathing-organ is at most a 

 specialised part. . . . In its most reduced form the shell 

 is only a hollow cone or plate, protecting the breathing-organ 

 and heart, as in Limax^ Testacella, and Carinaria. Its peculiar 

 features always relate to the condition of the breath ing- organ, 

 and in Terebratula and Pelonaia it becomes identified with the 

 gill. In the Nudibranchs the vascular mantle performs, wholly 

 or in part, the respiratory office. In the Cephalopods the shell 

 becomes complicated by the addition of a distinct, internal, 

 chambered portion (phragmacone), which is properly a visceral 

 skeleton " (Woodward). In a great many of the Mollusca 

 proper the shell consists of but a single piece, and they are 

 called " univalves." In many others the shell consists of two 

 separate plates or "valves," and these are called "bivalves." 

 In others, again, as in the Chiton, the shell consists of more 

 than two pieces, and is said to be " multivalve." Most, how- 

 ever, of the multivalve shells of older writers are in reality 

 referable to the Cirripedia. 



All the testaceous Mollusca (except the Argonaut), and most 

 of the " naked " forms, acquire a rudimentary shell before their 

 liberation from the ovum. In the latter this rudimentary shell 

 is cast off as the embryo grows, but in the former it becomes 

 the " nucleus " of the adult shell. In the bivalves the embry- 

 onic shell or " nucleus " is situated at the beak or " umbo " of 

 each valve, and is often very unlike the remainder of the shell. 



In composition the shell of the Mollusca consists of carbonate of lime 

 usually having the atomic arrangement of calcite with a small proportion 

 of animal matter. In the Pholadidce, however, the calcareous matter exists 

 in the allotropic condition of arragonite, which is very much harder than 

 calcite ; and there are many Gasteropods in which the shell is similarly com- 

 posed of arragonite. As regards their texture, three principal varieties of 

 shells may be distinguished viz., the "porcellanous," the " nacreous, " and 

 the "fibrous." In the " nacreous " or pearly shells, as seen in "mother-of- 

 pearl," the shell has a peculiar lustre, due to the minute undulations of the 

 edges of alternate layers of carbonate of lime and membrane. The " fibrous " 

 shells are composed of successive layers of prismatic cells. The " porcellan- 

 ous " shell has a more complicated structure, and is composed of three layers 

 or strata, each of which is made up of very numerous plates, "like cards 

 placed on edge." The direction in which the vertical plates are placed, is 

 sometimes transverse in the central layer, and lengthwise in the two others ; 

 or longitudinal in the middle, and transverse in the outer and inner strata. 



All living shells have an outer layer of animal matter, which is known as 

 the ' ' epidermis, "or " periostracum. " This is sometimes of extreme ten- 



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