34 Moll . 
MOLLUSCA. 
Haller (157) gives a detailed histological account of the nervous 
system of the marine Bhipidoglossa. The ganglionic cells have processes 
of three kinds : “ connecting process,” uniting two cells directly ; “ plexi- 
form process,” breaking up in the central nervous system ; “trunk process,” 
directly continuous with a nerve-fibre. Large triangular ganglionic cells 
are best seen in Haliotidce and Trochidce, two upper processes of which 
are always connected with smaller cortical cells, the lower being a plexi- 
form or trunk process. There are, in addition, “ central ” cells with no 
processes. No giant-cells, like those of Pulmonata and Opisthobranchiata , 
are present. The nucleus is always rounded, never reniform as in Pul¬ 
monata. The ganglion-cells of the central system have a thin coating of 
connective tissue, and between the protoplasmic particles is a yellowish 
pigment soluble in alcohol. The nervous investment is a continuation of 
the neurilemma, and is continuous over the whole central system. The 
connective tissue presents variations in one and the same species, a fact in 
accordance with Brock's views as to its variability in Mollusca. The 
nuclear portion of the central nervous system is a very delicate plexus, 
and neither dotted substance nor neuroglia. The pedal cord has a groove 
which is constant, but of no morphological significance ; the cords consist 
of two layers. The central ganglia have undergone concentration in 
Fissurella , but not in most Rhipidoglossa. The large pyriform and 
triangular cells are absent. The central nervous system and peripheral 
ganglia consist in general of marginal cells and a central plexus, formed 
from the processes of the cells and not from the neurilemma. Peripheral 
nerves arise from the cells and from the nerve plexus. 
The nervous system of Fissurella consists of two cerebral and two 
pedal ganglia and an “ asymmetric centre ” composed of five ganglia. In 
addition there is a triangular mass, which is probably a prolongation of 
the pedal ganglia and the two first of the asymmetric system. Panno- 
phorus furnishes a connecting link between Haliotis and Fissurella ; 
whilst Emarginula might be placed between the last and Parmophorus. 
Boutan (45). 
The nervous system of Tethys leporina has been dissected by Lacaze- 
Duthiers (226). It consists of the three groups of ganglia usual in 
Gastropods ; the nervous cells are contained in pyriform sacs, so that 
the ganglia have a racemose appearance, but after maceration six second¬ 
ary groups may be distinguished, and the triangular arrangement of 
three ganglia on either side may also be made out, although the connec¬ 
tives are extremely short. A careful investigation further reveals the 
asymmetric system in its normal connections. 
Jourdain (204) finds that, whilst in the adult Limacina there is a 
great concentration of the post-oesophageal nerve-masses, in the embryos 
they are but slightly developed, and connected by a long transverse 
commissure. The author differs from Lacaze-Duthiers, as he does 
not find that the otocyst is always primitively connected with the cerebral 
ganglia. The auditory nerve takes origin from a small ganglion situated 
on the connective between the post-oesophageal and pedal ganglia, but 
afterwards becomes connected with the pedal ganglia. The otocyst is 
not regarded as an organ for the perception of sound, but of disturbances 
