THYTIVOROUS MOLLUSCA. 829 



the animal's taste ; and on its sides a pair of lobulated sali- 

 vary glands, or sometimes two pairs, Avliich have each a 

 single excretory duct to convey their peculiar secretion into 

 its upper part, to lubricate and soften the mass. The gullet 

 is a muscular canal, lined interiorly with a mucous coat, 

 presenting, indeed, the same structure as the whole ali- 

 mentary canal, and is generally plaited in a longitudinal 

 direction. But the variety exhibited in the form, structure, 

 and disposition of the stomach and intestines is too great to 

 permit us to attempt a general view. The former is some- 

 times merely a membranous bag, or simple dilatation ; some- 

 times there is a gizzard, analogous to the gizzard of birds,* 

 to triturate the food previously to its transmission into this 

 bag ; sometimes there is a series of dilatations, or stomachs, 

 three or four ; and, again, in others we find the gizzard, or 

 stomach, armed with horny teeth, or laminee, of which the 

 Scylla?a affords a remarkable instance. -j- " There is no di- 

 vision of the alimentary canal into small and large intestines, 

 as in the higher classes ; or rather, among the mollusca, the 

 relative size of the different parts is reversed. Here the 

 pyloric extremity is usually the largest, while the anal is 

 more slender." J The intestine is usually a simple canal, 

 which, after making a volution or two among the lobes of 

 the liver, returns upon itself, and opens outwardly on the 

 side towards the anterior part of the body ; but, in Doris, 

 the anus is pierced on the back towards the tail ; and we 

 find, in Chiton and Dentalium, other exceptions to the usual 

 course ; for, in them, the intestine is straight, and has a pos- 

 terior tenuinal aperture, like the annulose tribes. To show 

 to what extent the alimentary canal is varied in this tribe, 

 let me give you a representation of that of the Tethys (Fig. 

 64), which you will compare with that of Pleurobranchus 

 (Fig. 65^) ; and, again, contrast these with the same parts in 

 Patella (Fig. 66). That these contrasts in structure are ac- 



* Of the stomach of Limnens Swammerdain, says, " It is of tlic same 

 structure, in all respects, with that of the hen or cock kind ; so that one 

 would think the real stomach of a hen is here represented, without any diifcr- 

 cnce, but that it is much smaller." 



t Cuv. Mem. 10, tab. 1, fig. 6. t Fleming Phil. Zool. ii. 411. 



§ " In the Pleurobranchus, the cesophagus dilates into a membranous 

 crop (fig. 65, a), at the lower part of which (b) the bile is poured in. It 

 communicates by a narrow cardia with the second stomach (c), which is a 

 gizzard with thin but muscular parietcs. The third stomach (d) is membra- 

 nous, and precisely resembles the plicated stomach of ruminants, in beino" 

 disposed in large but delicate folds, by means of which the alimentary 

 matter contained in it is moulded into long whitish cords. The fourth 

 stomach (e) is membranous, like the crop, but smaller. It is remarkable 



