388 



GASTEROPODA. 



interior armed in a manner which is probably 

 unique. The sides of the gizzard (b) are 

 covered with pyramidal plates of a rhomboidal 

 figure, the apices of which resemble the 

 tubercles found upon the grinding surfaces of 

 the human molar teeth. Of these there are 

 twelve larger plates arranged in quincunx, 

 besides several smaller ones placed near the 

 entrance of the organ. These teeth are of a 

 horny nature and formed of laminae parallel to 

 their bases : their adhesion to the surface of 

 the lining membrane is so slight that they are 

 detached by the slightest effort, without leaving 

 any trace of membrane or other bond of union, 

 the place of their attachment being only indi- 

 cated by a smooth and prominent surface, 

 corresponding in shape to the base of each 

 tooth. The apices of all these teeth meet in the 

 centre of the gizzard, and whatever passes 

 through that cavity must be bruised by their 

 action. 



The third stomach (d) is armed with teeth of 

 a totally different nature. These are little 

 conical hooks (c) attached to one side of the 

 organ only, and as little adherent in the dead 

 animal as are the pyramids of the gizzard 

 towards which their points are directed. In 

 the figure many have fallen off, leaving slightly 

 elevated spots indicative of the place of their 

 attachment. Near the pylorus is a large aper- 

 ture communicating with a ccecum of consider- 

 able size (y), evidently identical with the 

 spiral cfficum of the Cephalopoda both in its 

 position and relation to the insertion of the 

 biliary canals (e\ forming, as in Fishes, the 

 rudiment of a pancreas. From the orifice of 

 the ccecum a ridge is prolonged into the com- 

 mencement of the intestine (g). 



Accessory glands. The auxiliary chylopoietic 

 secretions found in the Gasteropoda are gene- 

 rally only two, the salivary and the hepatic. 

 In some rare instances already adverted to, as 

 in Doris and Aplysia, we may likewise add 

 the pancreatic furnished by the cceca, which in 

 those genera terminate in close vicinity with 

 the ducts issuing from the liver, and which, 

 from every analogy, represent the pancreas of 

 vertebrate animals. 



The salivary glands are constantly present 

 and seem to present a size and importance 

 corresponding with the mode in which the 

 mastication of the food is accomplished. In 

 those genera which are provided with a cutting 

 apparatus placed in the mouth, they are very 

 largely developed, as also in most of the 

 pioboscidean species, and it is only in the 

 Cyclobranchiate order, where the long spiral 

 tongue is used rather for the abrasion than the 

 mastication of the food, that they become small, 

 and, in a very few instances, undistinguishable. 

 In^/rg. 190, which represents the viscera of the 

 Snail, these glands are marked with the letters 

 *, and this engraving will give a good idea of 

 the general structure which they present, and of 

 the ordinary termination of the ducts which pour 

 the saliva into the oral cavity. The glands 

 are placed along the sides of the stomach, 

 which they partially invest, and sometimes 

 those of the opposite sides are intimately united 



with each other; their colour is whitish and 

 semi-transparent, and they are formed of small 

 lobes, which, in many species where their 

 texture is less compact, may be distinctly seen 

 to be formed of the ramifications of their 

 arborescent ducts, each ultimate division of 

 which is term mated by a secreting granule. 

 In Vaginulus (Jig. 189) the salivary glands are 

 small, but in addition to the ordinary struc- 

 ture (J') we find an additional tube or slender 

 ccecum (*/'), which, lying at first upon the 

 stomach, passes through the nervous collar to 

 join the duct by which the saliva is discharged. 

 The secondary divisions of the ducts gradually 

 unite to form an excretory canal for each of the 

 two glands, which invariably pour the salivary 

 secretion into the mouth in the vicinity of the 

 tongue. When very small, as in Testacella, 

 Onchidium, and Haliotis, they are found to be 

 merely arborescent tufts placed on each side of 

 the oral mass. In all the Pectinibranchiate order, 

 where the mouth is converted into a protrusible 

 proboscis, the glands themselves (Jig. 182, i) 

 are found within the visceral cavity, and their 

 ducts (e, e) are very long and tortuous so as to 

 follow the movements of the proboscis in which 

 they are lodged, running in contact with the 

 oesophagus to open at the extremity of that 

 tube on each side of the spiny tongue ; it is 

 even probable that the secretion which they 

 furnish at that point may assist, in some mea- 

 sure, in the destruction of the shells and other 

 hard bodies, which are submitted to the con- 

 tinued action of this organ. 



In Darin and P leu.ro branch us a glandular 

 structure of considerable size is found near the 

 commencement of the oesophagus, which is of 

 a brownish colour and plentifully furnished 

 with bloodvessels. This has been looked upon 

 as an auxiliary salivary organ, but as its duct 

 has not been as yet satisfactorily traced, its real 

 nature is unknown : but in Junthma there are 

 distinctly four salivary glands, each furnishing 

 a distinct duct; two of these run, as in Bucci- 

 num, to the extremity of the proboscis, whilst 

 the other pair empties the secretion of the 

 corresponding glands into the commencement 

 of the oesophagus. 



Biliary system. The liver throughout the 

 whole class is of great comparative size, en- 

 veloping the convolutions of the intestines and 

 filling a large portion of the visceral cavity. 

 That of the Snail consists of four large lobes 

 (fig- 190, h), each divisible into lobules, and 

 these again into secreting granules, from each 

 of which issues an excretory duct. The ducts 

 gradually unite into larger trunks, so that the 

 whole organ, when unfolded, accurately repre- 

 sents a bunch of grapes, the stem of which would 

 be the common biliary duct. In the same ani- 

 mal the excretory ducts from each of the divisions 

 of the liver unite into one canal, which opens into 

 the pyloric extremity of the stomach (g) in 

 such a manner that as much bile must be 

 poured into the stomach itself as into the com- 

 mencement of the intestine. In the Slug the 

 liver consists of five lobes, and from these are 

 derived two distinct biliary canals, which open 

 separately into the intestine, one on each side 



