370 ORGANS OF DIGESTION. 



at the posterior end of the trunk, it commonly opens, along 

 with the genital organs, on the right side nearer the anterior 

 extremity of the body. This lateral termination of the ali- 

 mentary and genital organs in the gasteropods, and also the 

 lateral position of their heart and respiratory organs, accord 

 with the want of bilateral symmetry remarkable in the lower 

 mollusca when contrasted with the articulated classes. 



In the doris which feeds, like most of the naked gas- 

 teropods, on marine plants, the mouth is furnished with 

 a pair of broad labial tentacula, as in the haliotis, and 

 resembling those of bivalved mollusca, besides the ordinary 

 vertical cephalic pair, and, though destitute of jaws, it is 

 provided with a lengthened tongue extended through the 

 pharynx. The cartilaginous surface of the tongue is 

 covered with minute sharp recurved spines, as in most other 

 phytophagous species destitute of jaws, and the short wide 

 muscular proboscis and pharynx lead to a long, wide and 

 tortuous oesophagus. Both follicular arid conglomerate salivary 

 glands pour their secretions into the mouth, and the capacious 

 round membranous stomach is perforated at its pyloric 

 portion with the numerous wide ducts of a large enveloping 

 liver, and with the oblique opening of a large single pan- 

 creatic follicle which is wanting in some of the species. 

 From the stomach the wide intestine passes around the 

 left side of the liver to the posterior end of the abdomen 

 where it perforates the auricle of the heart and opens on the 

 dorsal aspect of the body in the space surrounded by the 

 branchiae. Close to the anus, in this branchial space, there 

 is likewise the opening of an excrementitious gland im- 

 bedded, like the ink-gland of the octopus, in the substance 

 of the liver; there is a small round sac developed on the 

 duct of this renal organ, like a urinary bladder. The aplysia 

 faciata (Fig. 121) feeds, like the doris 9 on coarse marine 

 plants, and therefore presents a complicated condition of 

 all the chylopoietic viscera. Anterior to the long conical 

 cephalic tentacula are a pair of minute dark coloured eyes, 

 and the broad labial tentacula (121. a.) are moved by strong 

 muscular bands (121. b.) The lips are supported by two 

 cartilaginous laminee, and the tongue is covered with minute 

 recurved teeth. The wide muscular cavity of the mouth 

 (121. c.) receives the terminations of two lengthened fol- 

 licular salivary tubes (121. e. .), and lies over a large 



