34 



Ovo-testis or hermaphrodite gland embedded in liver, consisting of 

 tubes in which ova and spermatozoa are developed. 



The Hind end of one of the tubes with its contained ova and sper- 

 matozoa shown in fig. 12. 

 Duct of ovo-testis. 

 Albumen-gland forming an albuminous secretion. 



i Vas deferens or duct for conveying spermatozoa, opening into penis. 

 EEPRODUCTIVE ORGANS Oyiduct for conveying ova> ending in yagina 



Spermatheca or receptaculum seminis (L. receptacle of the semen) 



opening into vagina. 

 Penis with long flagellum (L. a whip), an eversible involution of 



the integument. 



Dart-sac eversible, with pointed dart ) ,. ., ,. ., 



} peculiar to Snail family. 

 Mucous glands J 



CIRCULATORY SYSTEM 



DIAG. 2. COMPARATIVE VIEW OF CENTRES OF CIRCULATION in 



a. Worm Dorsal trunk with transverse vessels discharging into it. 



b. Anodon Dorsal trunk dilated=the ventricle, and transverse vessels reduced to a pair forming 



the two auricles. 



c. Snail Dorsal trunk bent upon itself =the ventricle sending blood to anterior end of body by 



cephalic artery, and to posterior end by abdominal artery. Transverse vessels reduced to 

 one, the right, forming the auricle. 

 FIG. 8. COLOURLESS CORPUSCLES OF THE BLOOD. 

 NERVOUS SYSTEM AND SENSE ORGANS 



FIG. 9. NERVOUS SYSTEM forming a ring round gullet 

 Cerebral or supra-cesophagcal ganglia. 



, ,. ( Pedal, anterior and united with cerebral by commissures. 



Sub-ccsophageal ganglia < _ ' , . .^ , .,. 



( Parieto-splanchnic united with cerebral by commissures. 



FIG. 10. EYES, a pair, placed at end of tentacles 



Sclerot'ic (Gr. sMeros, hard) passing into a cuticular cornea. 



Choroid, the pigmented layer. 



Optic nerve expanding into an outer retina, the fibres of which pass through the choroid to form 

 an inner retina with rods and cones nearest the light. 



Vitreous humour (not shown). 



Lens. 



Cornea with a conjunctiva on its outer surface. 

 FIG. 11. ORGANS OF HEARING, a pair close to pedal ganglia 



Auditory canal. 



Sac containing fluid and otoliths. 



Auditory nerve from cerebral ganglia (not shown). 

 DEVELOPMENT 



DIAG. 3. EARLY STAGES OF DEVELOPMENT OF A VELIGEROUS GASTEROPOD, no evident velum in Snail 



Ciliated velum, an expansion of the integument about the head. 



Foot developing, with operculum (L. a lid) upon it. 



Shell external 

 CLASSIFICATION 



Sub-Kingdom Mollusca. 



Natural Order Odonto'phora, because it has a distinct head ; and an Odontophore. 



Genus Helix (Gr. a spiral), from the spiral form of the univalve shell 



Common Name SnaiL 



