BRITISH NUDIBRANCHIATE MOLLUSCA. 19 



hepatic glands of the branchial papillae, and probably represent, along with the ganglia from 

 which they originate, a portion of the gastro-hepatic plexus of the Dorides. 



The Senses. All the Nudibranchs are provided with auditory capsules, which contain 

 numerous vibratile otolithes in all the genera, except Fmbletonia and a portion of Folia 

 (Cavolina and Tergipes), where there is only a single large, spherical otolith e ; the auditory 

 organs thus, in these, retaining their embryonic condition. Eyes are also universally present, 

 and are only a little inferior in organisation to those of the higher gasteropods. The dorsal 

 tentacles are the organs of smell, and, judging from their great development, this sense must 

 be more acute in most of the Nudibranchs than it is in any other Mollusk, with the 

 exception, perhaps, of Nautilus. Olfaction, however, in these animals, probably is not so much 

 to assist in the discovery of alimentary matters, as to give warning of the unhealthy state of the 

 surrounding medium, arising from putrescence or other causes; for the Nudibranchs, 

 breathing more or less by the whole surface, and being entirely unprovided with covering 

 of any kind, are exceedingly liable to be affected by external influences. The sole object of 

 vision appears to be that of ascertaining the presence of light, and thus directing the animal 

 in its search for shelter in dark and concealed places. Touch undoubtedly resides everywhere 

 in the skin, but is specialised in the oral tentacles and parts about the mouth. The lips and 

 channel of the mouth are probably the seat of taste. 



The Skin. This varies very much in thickness in the several groups ; it is delicate and 

 soft in the Eolididcs, and in most of the Polycerince ; while in the Tritoniadce and Dorides, it is 

 thick and coriaceous, and generally more or less roughened with tubercular excrescences- 

 In the Dorididai it is always stiffened with imbedded calcareous spicula of various forms, which 

 in the Polycerince are for the most part scattered, and not very numerous ; but in the Dorides 

 are generally abundant, much crowded, and always more or less symmetrically arranged. 

 The skirt consists of a layer of muscular fibres, covered by a tegumentary envelope or cutis, 

 which is provided with an epithelium. The epithelium of the whole surface, not even 

 excepting the pedial disc in the Dorides, and perhaps in the other members of the order, is 

 provided with vibratile cilia. The dermal layer is thin and continuous with the inner or 

 muscular layer, which is amply supplied with muscular fibres, principally longitudinal and 

 transverse. Outside of, and amidst this muscular stratum, which is densest next the viscera, 

 is the cell-work or system of dermal sinuses, through which the blood flows on its return to 

 the heart. 



The dermal layer appears to secrete the tenaceous fluid that so abundantly exudes from 

 these animals; though that which lubricates the foot is probably provided by a special 

 gland. The two large glands in Fiona, which lie beneath the stomach, and the ducts of 

 which pass into the anterior margin of the foot, seem to be for this purpose. The so-called 

 salivary glands of Doto are, in all likelihood, of a similar nature (see note, p. 12) ; and Jfyirus 

 has a glandular body placed transversely under the channel of the mouth, which appears to 

 open externally in front of the crawling disc. Goniodoris is likewise furnished with a similar 

 gland, less perfectly developed, and the thickened margin of the foot of Polgcera ocellata 

 contains numerous secreting follicles. The thickening of this margin, the transverse groove, 

 and lateral prolongations, so common in these animals, are perhaps universally connected with 

 the production and suffusion of the mucus over the pedial disc. 



The cloak or mantle, which is characteristic of the Doridida and Tritoniadce, and 



