32 MOLLUSCA. 



or aperture, which leads into the least and usually largest turn of the 

 spiral ; its lip (peristoma), swollen in the adult animal, lies on the edge 

 of the mantle. The spiral is twisted to the right or left round an 

 axis which is directed from the apex to the aperture, and is indicated 

 either by a solid spindle (columella) or a hollow canal. When the 

 turns of the spiral are far removed from the axis, this canal may 

 become an almost conical space with a wide opening (Solarium). 

 The turns are usually closely applied to one another; more rarely 

 they are separated (Scalaria pretiosa). According to the position of 

 the columella, a columella edge or inner lip and an outer edge or outer 

 lip of the aperture may be distinguished. The latter may be entire 

 (holostomatous), or broken by an excavation which is often prolonged 

 into a canal (siphonostomatous). In many Gastropods an operculum 

 is added ; this is usually placed on the hind end of the foot, and 

 closes the shell aperture when the animal is retracted. Many terres- 

 trial Gastropods secrete before the beginning of the winter sleep an 

 operculum, which is thrown off again in the spring. 



The slimy integument consists of a superficial layer of cylindrical 

 cells, which are frequently ciliated, and of a connective tissue dermis, 

 which is inseparably connected with the dermal muscles. Cal- 

 careous and pigment glands are placed in the integument ; they are 

 especially numerous at the edge of the shell, where they contribute to 

 the growth and peculiar colouring of the shell. The shell, which is 

 a cuticular structure, is secreted by the epithelium, like other cuticular 

 structures , it becomes hard when the calcareous salts which are 

 mixed in the organic basis assume a hard and crystalline condition. 

 The superficial layer of the shell often remains uncalcified as a thin 

 delicate epidermis, while the inner surface is thickened by mother-of- 

 pearl layers (secreted by surface of mantle). The connection of the 

 animal to its shell is effected by a muscle, which, on account of its 

 position on the spindle (columella), is called the spindle muscle. This 

 muscle arises from the dorsal part of the foot, and is attached to 

 the spindle at the beginning of the last turn of the spiral. 



The nervous system presents a great resemblance to that of the 

 Lamellibranchiata, but there are many differences in detail. 



In the Placophora, whose nervous system presents close relations 

 to that of Neomenia and Chcetoderma, the ganglionic swellings are 

 not marked (fig. 495). In all other cases the three typical groups 

 of ganglia are present. *The cerebral ganglia (fig. 497, C g) are 



* The subjoined account of the nervous system is slightly modified from the 

 German. ED. 



