302 LECTURE XXII. 



and pectinated in all the dioecious Gasteropods, as the name of their 

 order indicates : in these they never exceed two in number, which 

 are of unequal size. In a few genera of amphibious Gasteropods a 

 pulmonary sac is combined with branchial organs. The branchial 

 surface is ciliated in all the Gasteropods, as is also the exterior surface 

 of the body in the small fresh-water species. 



Besides the large and well developed hepatic and salivary glands 

 which are associated with the alimentary canal, we have seen that cer- 

 tain fucivorous Gasteropods present the simplest rudimental condition 

 of the pancreas. The situation of the follicular urinary gland and of 

 its excretory duct has already been pointed out in the Paludina 

 vivipara : in some species the duct dilates to form a small recep- 

 tacle. A group of follicular glands, sometimes imbedded in a distinct 

 glandular sac, is present in many species for the elimination of some 

 peculiar and characteristic colour; the yellow liquid of the JBiillce, 

 and the famous purple secretion of the Purpura, are products of 

 saccular modifications of this follicular gland. Numerous simple and 

 scattered follicular glands lubricate the mantle with its characteristic 

 mucus in all the Gasteropods. 



At the grade of the Molluscous organisation which the Gasteropods 

 have reached, their capabilities and spheres of action become more 

 extended and diversified than in the Pteropods and Acephala; some 

 are terrestrial, some arboreal, whilst the more numerous aquatic 

 species are endowed with power to attain, subdue, and devour organ- 

 ised matter, dead and living. The nervous system of the Gastero- 

 pods is accordingly not only more complex and concentrated, not 

 only subordinated to better developed masses in connection with 

 organs of special sense and exploration, but it offers greater variety 

 in its general arrangement and especially in the position of its gan- 

 glions, than in the Lamellibranchiate class ; and with these modi- 

 fications considerable differences in the outward configuration of the 

 body are associated. 



Some Gasteropods, for example, are symmetrical, more or less flat 

 or depressed ; others are compressed ; the majority are contorted and 

 lose their symmetrical form in an oblique twist : there are other di- 

 versities of organic structure which more immediately affect the con- 

 dition of the nervous system, for some species possess both eyes and 

 tentacles, whilst others are blind and akerous. 



In the limpet (Patella), and in the Bulla, we find that the cerebral 

 ganglions, as in the bivalves, are still distant from each other, and 

 situated at the sides of the oesophagus, connected together by a 

 nervous chord or commissure which arches over that tube : from 

 these ganglions two filaments proceed backward on either side ; the 



