50 ZOOLOGY. 



sense, as it included radiata, like Actinia, Medusa, and Echinus ; Crustacea 

 like Lernsea, as well as Ascidia, Limax, and the Cephalopoda, and even 

 some annelida ; whilst his order (Vermes) Testacea included most of the 

 univalve and bivalve shell-fish. The " class Mollusca" of Lamarck, follows 

 liis class Conchifera, and includes the five " orders," as he terms them, of 

 Pteropoda, Gastropoda (excluding the spiral univalves), Trachelipoda 

 (including the spiral univalves). Cephalopoda, and Heteropoda. Pallas and 

 Cuvier were the first to use the term Mollusca in its modern sense ; and the 

 latter, in 1798, recognised three sections, subsequently provided with the 

 systematic names of Cephalopoda, Gastropoda, and Acephala. The term 

 Mollusca having by these means acquired a somewhat indefinite meaning, 

 Blainville proposed that of Malacozoa, which is the preferable term, if the 

 term Malacology supersedes that of Conchology. 



The shell of the Malacozoa can have one, two, or more pieces ; it may 

 be external or internal, and it varies much in size, being in some cases 

 capable of containing all the soft parts, with room to spare, and in others a 

 mere excrescence upon some part of the animal. It is secreted by the 

 mantle, a fold of the integument which appears as a flap, lining the shell in 

 bivalves, and a continuation of it appears as a collar around the neck, and 

 lining the aperture of the shell in the spiral univalves. In the order 

 Tunicata the body is inclosed in a kind of coriaceous purse instead of a shell. 



The mantle usually secretes two kinds of material, the nacre, or pearly 

 portion of the shell, and the epidermis, ov periostraca ; but in some genera 

 the latter is absent, as in Oliva {2)1. To, jigs. 122, 125, 126) and Cyprcm 

 {2)1. T6, figs. 5-7), in which the wide mantle is turned up on each side, so 

 as to inclose the shell. When the shell is broken by accident, the crevice 

 is closed by layers of nacre; and if a pebble gets into the shell by accident, 

 and cannot be got rid of, it is covered in like manner. When the nacre 

 or " mother-of-pearl" is of a fine quality, the tubercular masses which arise 

 from injuries have a peculiar lustre, which causes them to be admired under 

 the name of pearls. "Shells are often found repaired, which had been 

 crushed and distoiied to such an extent as to lead one to believe it scarcely 

 possible for an animal to survive after so nnicli mutilation. 



The varied spots and lines which ornament so many marine shells, are 

 distributed by the periodical action and inaction of the secreting process. 

 The species of the genus Conus, although covered by a thick epidermis, are 

 usually marked with bands, spots, and reticulations of brilliant colors, which 

 make them a favorite ornament to collections. The epidermis, however, 

 should not be removed, or it should at least be preserved upon a specimen 

 of each species, as it varies considerably according to the species. T]ie 

 former unscientific custom of polishing sucli shells as have a homely 

 exterior is now discontinued. 



The microscopic structure of shells presents many curious features, which 

 have been thoroughly studied by Carpenter, who has illustrated the subject 

 with figures. In some shells the structure resembles that of minerals, and 

 seems to be formed of minute crystals of carbonate of lime, or of fibres 

 resembling arragonite. 

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