232 



SYSTEMATIC ZOOLOGY 



On the dorsal side of the body is the mantle, consisting of a 

 single caplike fold of the integument lining the shell. In some 

 groups there is no shell, in others it is a thin, delicate, chitinous 

 structure, but usually it is hard, calcareous, with a pearly lining. 

 The shape of the shell varies greatly, from a simple conical cap 

 to a greatly twisted spiral. The shell is generally large enough 

 so that the foot and head can be completely withdrawn inside, 

 and in some groups there is a flat, chitinous plate attached to the 

 foot, which completely closes the aperture to the shell when the 

 foot and head are within ; this plate is called the operculum. 



Fig. 234. Patella, a limpet; trochosphere larvae in different stages. A, with circular blas- 

 topore and two foot elevations; B, with elongated blastopore and two mesodermal bands. 

 (After Patten, from Parker and Haswell's Text-book.) 



The mantle cavity contains one or two gills in the aquatic Gastro- 

 poda, and receives the anus and the openings from the excretory 

 and reproductive organs ; in the male it contains the copulatory 

 organ, which is generally well developed. In the terrestrial 

 genera the mantle cavity is transformed into a sort of lung or 

 respiratory chamber. 



Excepting the head and the foot, the body is to a consider- 

 able degree asymmetrical. The development is attended by the 

 formation of larvae ; the first stage is a trochosphere (Fig. 234), 

 which is succeeded by the veliger (Fig. 235), in which a ciliated 

 membrane or velum is present on the head and forms the organ 



