1894- NOTES AND COMMENTS. 411 



laid, and in the same spot in which they are deposited — the crack of 

 a rock or the interior of an empty sheh — and he further notes that they 

 do not feed while sitting, so that they become quite thin. 



The marked peculiarity concerning the new species, which is 

 described by MM, Perrier and Rochebrune {Comptes Rcndus, vol. 

 cxviii., p. 770) under the name of Octopus digneti, appears to be that 

 it habitually spawns in empty bivalve shells (Cytherea, Pccien, etc.), 

 lining the interior of the two valves with its eggs, and itself occupying 

 the intervening space. Whether it merely remains in this situation 

 during incubation, or whether, hermit-crab- wise, it makes its abode 

 in empty shells a matter of permanent policy, does not transpire ; but 

 MM. Perrier and Rochebrune, judging from their concluding remarks, 

 evidently incline to the latter idea. In fact, this habit is merely a 

 specialisation of the ordinary habit possessed by these shell-less 

 Cephalopoda of protecting their soft bodies by inserting them in 

 foreign shells, rock crevices, old tins, or whatever first presents itself. 



Nerves of Molluscs. 



A LONG and apparently most careful paper on the histology and 

 organology of the nervous centres of Gastropoda as exemplified in 

 the genera Helix, Arion, Zonites, a.nd Limax, from the pen of M. B. de 

 Nabias, has just appeared in the Actes dc la Socicte Linneenne de 

 Bordeaux (vol. xlvii., pp. 11-202, 5 pis.). 



As the author justly remarks, the microscopic structure of the 

 nervous system of the Gastropoda — indeed he might have said of 

 Mollusca generally — is almost totally neglected, attention being 

 devoted solely to the external topography. Mr. Nabias' researches 

 show that there exist two sorts of nervous cells in the nervous 

 centres. First, ganglionic cells properly so called, of variable size, 

 that occur in the suboesophagian centres, in the visceral ganglia, and 

 in the posterior portion of the brain, where they are disposed radially 

 round the central fibrillary mass. The larger cells are near the 

 periphery, the medium-sized and smaller ones in closer proximity to 

 the central mass. Secondly, small spherical cells of uniform size, 

 only met with in the anterior region of the brain (Protocerebron), in 

 the terminal ganglia of the tentacles (both upper and lower) and the 

 ganglion of the external labial nerve, which must be considered as 

 the gustator}' nerve. 



The number of cerebral nerves, amounting to nine pairs, are 

 constant for the five genera. M. Nabias distinguishes three regions 

 in the cerebral ganglion — protocerebron, mesocerebron and post- 

 cerebron — for the last of which the term metacerebron would have 

 been preferable. 



NULLIPORES. 



It is already well-known that the small calcareous marine Algae, 

 commonly termed " NuUipores," sometimes form great deposits of 



