308 LECTURE XXII. 



cealed within the visceral cavity, but which can be everted like the 

 finger of a glove and protruded externally. 



The ovarium is a larger, more elongated, and less minutely granular 

 body than the testis, to which it is inferior or anterior in position. 

 The oviduct is long and wide, with thick glandular tunics, and, near 

 its termination, it communicates with the short and wide ducts of 

 two ramified tubular glands, called the ' multifid vesicles.' But the 

 complexity of the generative apparatus does not end here : the snail 

 is provided with a pyriform muscular sac, the aperture of which ter- 

 minates close to the generative outlet. The expanded base or head 

 of a slender conical calcareous style or dart is attached to the fundus 

 of the sac : its sharp apex extends close to the orifice, and by the 

 contraction of the sac it can be protruded outwards. With it the 

 snails pierce each other's skin, and the function of this curious organ 

 would seem to be to cause a preliminary excitement to the reciprocal 

 union of the two androgynous individuals. 



In the dioecious Gasteropods the intromittent organ is usually of 

 extraordinary length : it is grooved in most, perforated in a few ; 

 capable of retraction in the Paludina, but doubled back upon the 

 outside of the mantle when drawn into the shell by the Biiccinum 

 and Strombus. In the Carinaria it is bifid. 



The ova of the marine Gasteropods are enveloped, before exclusion, 

 in mucous capsules, prepared by a special gland situated near the 

 termination of the oviduct. The secretion in some species is soft, 

 flexible, and transparent; in most it hardens by contact with the 

 sea water, and assumes various definite and characteristic forms : the 

 nidus is sometimes simple, sometimes compound, but each compart- 

 ment contains many ova; and the development of the embryo pro- 

 ceeds in the nidamental chamber until its own little defensive shell is 

 acquired. 



In the terrestrial Gasteropods the ova are usually spherical and 

 opake, and separately extruded. Snails and slugs oviposit in the 

 earth. The tropical Bulini * cement leaves of trees together to form 

 an artificial nest for their large eggs. 



The ova of the sea.-s\ug (Trifo?iia) are expelled together in the 

 form of a long thread, and are arranged in a spiral manner in the 

 tenacious transparent covering of the thread. In the Doris muricata 

 the ova are aggregated in a flattened spirally disposed albuminous 

 band when excluded from the oviduct. The harder albuminous cap- 

 sules which defend the ova of other marine Gasteropods ofl^er a great 

 variety of forms, some of which are remarkable for their complexity, 



* Prep. 2943. B. 



