242 ZOOLOGY. 



" head." In the bivalves (lamellibranchs) there is a very 

 slight cephalization, or collection of special organs about the 

 anterior end. For this reason they are often called Acalephs. 

 In the gasteropods (snails, etc.) and cephalopods (squid), on 

 the other hand, the head is well developed both as to special 

 mouth parts and nervous organs. 



The forms with shells are somewhat more limited in size 

 than the cephalopods, which furnish the largest representatives 

 of the phylum, measuring in extreme cases 20 to 40 feet in 

 the reach of the arms. 



The calcareous shell insures abundant fossil remains, repre- 

 sentatives being found in various geologic formations from 

 the beginning of the Palaeozoic era to the present. 



281. Integument (skin). This consists of a layer of epi- 

 dermal cells, covering a deeper dermal layer derived from the 

 mesoderm. The former is made up chiefly of the supporting 

 cells and the simple glandular cells which secrete mucus, or 

 lime, or pigment. In many forms a large portion of the epi- 

 thelium in the mantle cavity (as the inner surface of the 

 mantle and the covering of the gills in Lamellibranchs) is 

 ciliate. The dermis is a complex of connective tissue, muscle 

 fibres, pigment cells, etc. The mantle is a fold of the skin 

 which is very characteristic of Mollusca. It grows out from 

 the dorsal wall of the body and encloses a space known as the 

 mantle cavity. It is usually important in respiration, and con- 

 tains the shell-glands. 



282. Shells are formed in all the classes of Mollusca, al- 

 though naked forms occur in several of them. The shell is 

 a true secretion or excretion, deposited by the epithelial layer 

 of the mantle. It consists of three layers : (a) a thin external 

 layer of organic material known as conchiolin, (fr) the pris- 

 matic layer, embracing the greater thickness of the shell and 

 made up of prisms of carbonate of lime cemented by con- 

 chiolin, and (c) the nacreous or pearly layer over the inner 

 surface. The edge of the mantle secretes the first and second 

 layers, and they usually show lines of growth parallel with 



