132 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The ripe sperms pass down the spermiduct and into the Hagellum, 

 by a viscid secretion of which they are bound into a thread-like 

 packet (spermatophore). Two individuals mutually fertilize each 

 other, the eversible penis being used as a copulatory organ, by 

 which the spermatophore is conveyed into the sperm otheca. A 

 preliminary stimulus is given by the ejected darts, which may be 

 found sticking in the skin. After some days the spermatophore 

 disintegrates and the liberated sperms fertilize the ova as they 

 pass down the oviduct. Each oosperm is surrounded by an albu- 

 minous investment secreted by the albumen gland, and, external 

 to this, by a tough calcareous shell, with the formation of which 

 the mucous glands appear to be concerned. The eggs, thus con- 

 stituted, are about a quarter of an inch long in H. pomatia. They 

 are laid in damp earth during June or July. 



8. Muscular System. The foot is almost entirely made up of 

 muscle-bands, arranged in a complicated way, and bringing about 

 creeping movements by their contraction. The spindle-muscle 

 serving to pull the body into the shell is a firm band taking 

 origin in the columella, and, dividing into right and left halves, 

 again subdivided into numerous slips which are inserted into the 

 foot. The retractor of the buccal mass, by which the snail's 

 head is drawn in, is a branch of this muscle. The contrary 

 movement is effected by protractors of the buccal mass which 

 take origin in the foot, and there are also depressors, having a 

 similar origin, which pull the buccal mass down. A branch of 

 the spindle-muscle on each side constitutes a tentacular retractor 

 which bifurcates into two slips traversing the corresponding ten- 

 tacles and inserted into their tips. When these muscles contract 

 they draw the tentacles back into the body-cavity, invaginating 

 them. The retractor penis has been mentioned above (p. 131). 



The muscle-fibres are composed of slender spindle-shaped cells, 

 which, except in the odontophore, are unstriated. 



9. Nervous System (Fig. 35). This is remarkable for its great 

 concentration, and is chiefly localized in the head, where a nerve- 

 ring enclosed in a firm sheath is found 'surrounding the gullet, 

 immediately behind the buccal mass in the extended state. 



When the animal retracts itself, this is drawn backwards through the 

 nerve-ring, which is, therefore, then found further forwards than usual. 



The ring is thickened dorsally into two cerebral ganglia, connected 

 together by a broad commissure, and ventrally into a ganglionic 



