2/6 ELEMENTARY BIOLOGY. [CHAP. 



stance of which they break up into a second [renal) 

 capillary network. 



The blood contains amoeboid corpuscles, which float in 

 an opalescent serum ; it assumes a bluish tinge on exposure 

 to the atmosphere. 



The central nervous system is enclosed in a membranous 

 circum-oesophageal sheath. It consists of three yellowish 

 ganglionic masses ; the supra-ccsophageal or cephalic lying 

 above the gullet and giving off nerves to the head segment 

 and related parts ; the pedal which supplies the foot and 

 body-wall ; and the parieto-splancJinic whicli distributes 

 fibres to the body-wall and viscera, and all parts lying behind 

 its point of origin, irrespective of the foot. These ganglionic 

 centres are connected together by lateral commissures; and 

 from the cerebral mass there arise a system of buccal nen^cs 

 in relation with the buccal mass and its odontophore, and 

 others distributed to the sense organs. The latter are, a 

 pair of small auditory vesicles to be hereafter described (see 

 Sect. J. 3) and visual and tegumental sense organs borne 

 by the tentacles, to which reference has already been made. 



The snail is hermaphrodite and the sex-organs are highly 

 complicated. With the exception of the hermaphrodite 

 gland or ovoteslis, a portion of the duct of the same and its 

 appended albumen-secreting gland — all of which are lodged 

 in the visceral sac, they fill the greater part of the spacious 

 body cavity and can be at once recognized by their dead- 

 white colour. As the hermaphrodite duct approaches the 

 exterior it suddenly divides into distinct oviduct and vas 

 deferens ; the base of the latter is enlarged to form a swollen 

 eversible intromittent organ or penis, which opens, side by 

 side with the oviduct, into an integumental pit or genital 

 cloaca. Appended to the whole apparatus there are several 

 accessory glands and diverticula. Chief among these is a 



