PTEROPODA AND GASTROPODA. 561 



dorsal or lateral lobe in the Apneusta with those appendages. Some 

 peculiar scent-follicles must exist in the garlick snail {Helix aUiaria)- 

 A species of slug, called plwsphorax, is slightly luminous. 



Gastropods have the power of repairing injuries and of reproducing 

 lost parts to a considerable extent. New tentacula soon grow to 

 replace those which may have been amputated. When they support 

 eyes, as in the snail, the organs of vision are also reproduced : the 

 mouth, with the horny jaw, has grown again in this Gastropod; and 

 when the snail has been decapitated, but with the oesophageal gang- 

 lions left behind, the head has been restored. 



The general conditions of the sexual system have been already 

 briefly defined. The complexity and bulk of the combined organs in 

 the common slug and snail are truly extraordinary. The essential 

 organs, testis and ovarium, are associated together in the form of a 

 small, compact gland {fig. 207, q)*, composed of many parallel caeca, 

 imbedded in tlie substance of the liver, and occupying, in the snail, 

 the apex of the shell. Each ca3cum consists of an external layer, 

 producing ova, and an internal sac, folded in the first, producing 

 semen.f The walls of these invaginated sacs are usually in contact, 

 but become separated at the points, where the ova push the ovarian 

 sac outwards, and the sperm-cells the testicular one inwards. The 

 common ducts from these series of combined sacs are also invaginated, 

 the oviduct being external, the sperm- duct internal, and usually un- 

 dulated. The testicular C£eca and sperm-duct are lined with ciliated 

 eiDithelium : this is not present in the ovarian sacs. 



Both invaginated tubes (r) enter the albuminiparous sac {s\ and 

 separate where they quit that part : there is a dilatation or sperm- 

 reservoir, where the sperm-duct quits the base of the albuminiparous 

 sac, and this latter may be regarded as a more special dilatation of the 

 oviduct, with follicular walls. The sperm-duct, enlarged and with 

 more glandular walls {t\ now proceeds with many short folds parallel 

 with the uterus to the base of the penis. This is a long and slender 

 organ (v), usually retracted and concealed within the visceral cavity, 

 but, like the finger of a glove, capable of being everted and protruded 

 externally. The so-called " uterus " {u) is a long canal, with trans- 

 versely plicated glandular walls, terminated by a vagina opening into 

 the common genital vestibule, the external orifice of which is near 

 the mouth of the respiratory sac, on the right side of the head. 

 With the vagina there communicates the duct {z') of a small pyriform 

 vesicle {z), which is a sperm-reservoir, for the fecundating element of 

 another individual received in coitu. A small caecum is developed 



* CCCXIII. p. 370. t CCCLXIV. p. 378. 



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