414 GASTEROPODA. 



genera, the stomach is thus devoid of complication, it is by no 

 means unfrequently found to be provided with a powerful crushing 

 apparatus, that forms a strong gizzard adapted to bruise, cut, or 

 tear the food introduced into it. In Scyllaa, for example, this 

 gizzard, situated at the entrance to the stomach, contains twelve 

 horny cutting blades disposed around its interior, and arranged in 

 a longitudinal direction ; their sharp edges, therefore, meeting in 

 the centre, efficiently divide whatever passes between them to- 

 wards the proper digestive stomach. In Aplysia there is first 

 a capacious crop, then a strong gizzard studded internally with 

 pyramidal blunt teeth, and to this succeeds a third cavity armed 

 with sharp pointed hooks attached to one side of its walls, and so 

 disposed as to form a kind of carding machine by which the food 

 is still more effectually torn to pieces. 



Various modifications in the form and structure of these sto- 

 machal teeth are met with in the different genera of the GASTE- 

 ROPODA that possess such an apparatus; but whatever their 

 shape, size, number, or position, the office assigned to them i& 

 the same. 



(450.) The liver is proportionately of very large size in the 

 Mollusca we are now describing. Its composition is similar in all ; 

 being made up of bunches of secreting follicles united by the 

 branches of their excretory ducts, and kept together by means of 

 a delicate cellulosity and the ramifications of blood-vessels. We 

 have already described the hepatic viscera of the snail ; and the 

 liver of Buccinum, unravelled so as to show its intimate structure, 

 is represented in the preceding figure (j^g. 192, n, o, p), which 

 requires no additional explanation. 



But if the structure of the liver is similar in all the Gasteropod 

 Mollusca, the manner in which the bile is poured into the intes- 

 tine varies remarkably. The most ordinary position of the orifices 

 of the hepatic ducts is at the termination of the stomach, in the 

 vicinity of the pylorus ; as is the case in the majority of other 

 animals : but many exceptions to this rule are met with in the 

 class before us. 



In Scyllcea the bile is poured into the oesophagus just before it 

 terminates in the gizzard. In many genera the biliary canals open 

 into the stomach itself; and in one remarkable genus, Onchidium, 

 there are three distinct livers, each provided with its proper excre- 

 tory duct ; and, what is still more anomalous, these three glands, 

 which in every particular strictly resemble each other, unless per- 



