MOLLUSCA 247 



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 tlie 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. 



Fig 1 10. 



9 — i 



Fig. iio. Helix. A, an empty shell in section from apex to base. B, the relation of the 

 animal to the shell when extended, a, apex of shell; an., anus; ap., aperture of shell; c, columella 

 or axis of shell; e, ejrestalk;/, foot; I, lip of shell; m, edge of mantle, which secretes the shell; r.o., 

 respiratory aperture; s, suture, between the whorls; t, tentacles. 



Questions on the figures. — ^What suggestions of bilateral symmetry are 

 shown by the snail? Where does growth occur in the shell? What are the func- 

 tions of the tentacles? What is the function of the edge of the mantle called the 

 "collar" (m)? 



287. 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 ciliated. The 

 dermis is a complex of connective tissue, muscle fibres, pigment 

 cells, etc. The mantle is a fold of the integument which is very 



