48 Dr. Johnston on Scottish Mollusca, 



ture is rasped and frittered down, and in this dissolved state 

 the mass is again brought forward by the regulated contrac- 

 tions of the adjoining muscles, and the peristaltic motions of 

 the tongue itself, to be forced down the oesophagus into the 

 stomach, which is simply a membranous dilatation of the ali- 

 mentary canal. This canal is very short, scarcely equal to the 

 length of the body, a fact which may seem irreconcilable with 

 the unnutritive character of their fare ; but the want of length 

 is probably compensated by the unusual size and complexity 

 of the salivary glands, and perhaps also of the liver*, which 

 lies in the immediate proximity of the stomach, and more or 

 less envelops it, pouring in its copious secretion by several 

 apertures. In Doris there is another singular organ which 

 sheds its secretion into the stomach, a vesicle with the inner 

 surface roughened with conical papillae, but which has no di- 

 rect communication with the substance of the liver. Dr. Grant 

 considers this organ to be analogous to the pancreas in higher 

 animals f. 



The Nudibranchia are hermaphrodites of a peculiar kind : 

 each individual possesses the organs of both sexes, but is in- 

 capable of self-impregnation, and requires the aid of another 

 to render the ova fruitful. These are deposited on the under 

 sides of stones, on shells, and on the roots and fronds of sea- 

 weed, in glutinous masses, which are sometimes broad and flat 

 like a ribbon, and at other times more like ravelled thread. 

 The number of ova in each mass is prodigiously great, and 

 they are usually arranged in regular lines, straight or zig-zag, 

 each ovum, or perhaps two or three, inclosed in a separate 

 vesicle imbedded in the common mass, which is itself covered 

 by a membrane of the most perfect transparency. The embryo, 



beset are reflected, and draw the food to the gullet ; while in the former, 

 the spines are deflected, and serve to keep the food within the reach of the 

 aws. The tongue'of the Doris, therefore, serves for deglutition, that of the 

 Tritonia for mastication." — Phil, of Zoology, ii. 469- I have not noticed 

 this distinction ; and it should be remembered that the prickles are move- 

 able and may be directed either backwards or forwards, though the former 

 is their ordinary position. 



• According to Cuvier the liver is not comparatively large, at least in 

 Tritonia. — Mem. p. 12. But Blainville says that the liver has always ap- 

 peared to him to be larger in phytophagous than in zoophagous mollusca. 

 — Man. de Malacologie, 124. 



f Loudon's Mag. Nat. Hist. viii. 79. 



