318 MOLLUSCA. 



development, we should not know that any departure from the 

 original symmetry had occurred. In the Gastropoda, on the other 

 hand, not only is there one form in which there is no asymmetry 

 at all, but the other members of the group present the distortion 

 in various degrees, and in no living Gastropod has a new symmetry 

 been completely attained. It is possible, therefore, by a comparison 

 of adult forms to arrive at an understanding of the course by which 

 the distortion in the most extreme members has been brought about. 



The Mollusca are unsegmented Eftiimals without jointed append- 

 ages. The body is covered by a soft, slimy integument, bounded 

 externally by an epithelium frequently ciliated and containing a 

 considerable number of gland-cells. They lack both an internal 

 and external locomotory skeleton, and are therefore especially suited 

 for life in water. But few of them are terrestrial, and when 

 this is the case the movements are always limited and slow ; while 

 the aquatic forms may be endowed with the power of rapid 

 swimming. 



Beneath the epithelium is the dermal connective tissue, which 

 consists of cells of various forms, branched cells, elongated fibre- 

 cells, and especially characteristic are the vesicular cells, which 

 sometimes secrete and contain calcareous concretions or spicules 

 (Pleurobranchs, and some Nudibranchs). This tissue contains 

 blood-spaces, the filling of which with blood may cause a turgescent 

 swollen condition of that part of the integument. Sometimes it is 

 condensed, and forms the cartilage of the Cephalopoda, the skeletal 

 tissue of the branchial filaments of Lamellibranchs, the shell of the 

 Cymbuliidae. 



The muscular fibres are generally non-striped. There is often 

 an appearance of striation in consequence of the arrangement of 

 the granules, e.g., muscles of buccal mass, of heart, columellar 

 muscles of the larvae of some Nudibranchs, etc., and in some cases 

 the fibres are more distinctly cross-striped, e.g., portion of adductor 

 muscles of Pecten. 



The dermal muscular system plays an important part in the loco- 

 motion of these animals, especially that part of it which is placed 

 on the ventral surface of the body. In this region it is greatly 

 developed, and gives rise to a more or less projecting locomotory 

 organ of very various shape, the foot (Fig. 254). 



The foot always consists of an unpaired median structure, which 

 is sometimes divided into several parts, and may possess, in addi- 

 tion, paired lateral lobes the epipodia. 



