MOLLUSCA : GENERAL CHARACTERS. 277 



closely related to the respiratory organs ; "indeed it maybe 

 regarded as z. pneumoskekton, being essentially a calcified 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,;ZX\A Carinaria. Its peculiar 

 features always relate to the condition of the breathing-otgan, 

 and in Terebratula and Pelonaia it becomes identified with the 

 gill. In the Nudibranchs the vascular mantle performs, wholly 

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

 becomes comphcated by the addition of a distinct, internal, 

 chambered portion {pkragmacone), 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 be- 

 comes the "nucleus" of the adult shell. In the bivalves the 

 embryonic shell or " nucleus " is situated at the beak or 

 " umbo " of each valve, and is often ve_ry unlike the remainder 

 of the shell. 



In composition the shell of the Mollusca consists of carbon- 

 ate of lime — usually having the atomic arrangement of calcite 

 — with a small proportion of animal matter. In the Phola- 

 didce, however, the calcareous matter exists in the allotropic 

 condition of arragonite, which is very much harder than calcite. 

 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 car- 

 bonate of lime and membrane. The "fibrous" shells are 

 composed of successive layers of prismatic cells. The " por- 

 cellanous " shell has a more complicated structure, and is com- 

 posed of three layers or strata, each of which is made up of 

 very numerous plates, " like cards placed on edge." The di- 

 rection in which these vertical plates are placed, is sometimes 

 transverse in the central layer, and lengthwise in the two 



