474 Miscellaneous. 



portion, which is deeply invaginated in a very narrow caudal portion, 

 presents a complete analogy of form and dimensions with the appa- 

 ratus of fixation of the Bothriocephalus. — Comptes Bendus, Sept. 

 21, 1863, p. 569. 



On the Structure of the Nervous System in the Gastei'opodous 

 Mollusca. By Salvatore Trinchese. 



The types investigated by the author are Helix pomatia, Arion 

 rufus, and Lyinncsus stagnalis. 



In all the nervous centres of these animals there are — 



1. Round or pyriform cells, of variable dimensions, enveloped by 

 a thick sheath of conjunctive tissue. 



2. Small cells, of irregularly triangular form, round which no 

 envelope is perceived. 



3. Free nuclei like those met with in the grey substance of the 

 cephalorachidian system in the Vertebrata. 



In these animals there are no apolar or unipolar cells, and bipolar 

 cells are rare. The cells usually present four prolongations. Each 

 cell emits a prolongation to each of the cells surrounding it, whilst 

 other processes pass between the latter to other cells at a greater or 

 less distance. 



The nervous cells generally occupy the periphery of the ganglia. 

 The central portion of the latter is occupied only by nervous fibres 

 and conjunctive tissue. The nervous cells of one ganglion are never 

 all of the same dimensions or of the same form. The largest cells 

 generally form the stratum nearest the periphery, and the cells 

 diminish in size towards the centre of the ganglion. The deepest 

 stratum is formed of very small cells and free nuclei. This arrange- 

 ment shows that these elements are in a state of continual develop- 

 ment. 



The two cerebroid ganglia are formed, in their upper part, of large 

 round cells and of pyriform cells. These elements, which are ar- 

 ranged in groups, all emit processes which go to form nerves. The 

 large round cells are placed at nearly equal distances from each 

 other. In the intervals between the round cells there are constantly 

 pyriform cells, of which the processes cross. In the lower region of 

 the two cerebroid masses, very small triangular cells are observed. 



At the anterior part of these masses there are, in Helix and Arion, 

 four small ganglia, of the nature of those which have been described 

 under the name of accessory cerebroid ganglia. These are concealed 

 beneath the envelopes of the cerebrum, and can only be seen when 

 the latter are rendered transparent by reagents and the organ is 

 slightly magnified. Of these ganglia the two outer ones must be 

 called optic ganglia, as they give origin to the optic nerves. They 

 consist of free nuclei and of nervous fibres proceeding from the ante- 

 rior part of the cerebroid masses. The free nuclei alone occupy the 

 outer portion of the ganglion, and the nervous fibres the inner part ; 

 the line of separation is very distinct. The two inner ganglia are 

 composed of voluminous cells pressed against each other. 



On the course of the nerves connecting the cerebroid masses with 

 the pedal ganglion there is a small ganglion composed of cells united 



