

DIGESTIVE SYSTEM. 545 



liver, is contained in a kind of bag attached to the back of these sin- 

 gularly-formed animals, and in some genera, as for example Carinaria, 

 is defended by a delicate transparent shell, which in appearance offers a 

 miniature resemblance to the Argonaut. It is in this visceral sac that 

 the heart and generative apparatus are likewise generally enclosed ; but 

 in many forms of the Heteropoda both the appended sacculus and shell 

 are wanting, in which case the viscera are, of course, lodged in the general 

 cavity of the body. 



(1450.) But although in Bucdnum, Pterotrachea, and kindred genera 

 the stomach is thus devoid of complication, it is by no means linfre- 

 quently found to be provided with a powerful crushing apparatus, that 

 forms a strong gizzard, adapted to bruise, cut, or tear the food intro- 

 duced into it. In Scyllcea, 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 towards 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. 



(1451.) Various modifications in the form and structure of these 

 stomachal teeth are met with in the different genera of the GASTEROPODA 

 that possess such an apparatus ; but whatever their shape, size, number, 

 or position, the office assigned to them is the same. 



(1452.) 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 excre- 

 tory 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 Bucdnum, unravelled so as to 

 show its intimate structure, is represented in the preceding figure 

 (fig. 274, n, o, p}, which requires no additional explanation. 



(1453.) But if the structure of the liver is similar in all the Gaste- 

 ropod 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. 



(1454.) 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 excretory duct, 

 and, what is still more anomalous, these three glands, which in every 



