172 Chronicles of Science. | Jan. 
nervous system of the Gasteropodous Mollusca, taking as types the 
Helix pomatia, Arion rufus, and Lymnea stagnalis. He finds that 
in all the nervous centres of these animals there are,—round or pyri- 
form cells of variable dimensions, enveloped in a thick sheath of con- 
nective tissue ; small cells of irregularly triangular form, round which 
no envelope is perceived ; and free nuclei, like those met with in the 
grey matter of the cerebro-spinal axis in Vertebrates. The cells 
usually present four prolongations, passing to each of the cells sur- 
rounding them, whilst other processes pass between the latter to other 
cells at a greater or less distance. These cells are usually found in the 
peripherical portions of the ganglia, the interior being occupied with 
fibres and conjunctive tissue. The optic ganglia consist of free 
nuclei and nervous fibres proceeding from the anterior part of the 
cerebroid ganglia, two in number. On the anterior portion of them 
in Helix and Arion, there are four small accessory cerebroid gan- 
glia; and on the course of the nerves connecting the cerebroid masses 
with the pedal or abdominal ganglion, there is a small ganglion, 
composed of cells united in groups, like the compartments of an 
orange. The peripheral nerves are formed of very delicate tubes, 
having on their walls nuclei similar to those which are observed in 
the higher animals in the Embryonal state. Their mode of termi- 
nation in the muscles is remarkable. The nervous element on arri- 
ving at the muscular fibre, loses its proper wall, and the axis-cylinder 
alone penetrates the muscle, dividing into two very slender filaments. 
These take opposite directions, each traversing one-half of the mus- 
cular fibre, on arriving at the extremity of which they terminate in 
very fine points. 
Although we have long been acquainted with the young state of 
the true crabs, and hermits, under the form known as Zoéa, especially 
distinguished by the want of the ten feet to which the adult animals 
are indebted for their name of Decapoda, it is only recently that Fritz 
Miller has described the Zoéa forms of the Porcellane as approach- 
ing most closely to those of the crabs. He has now added the in- 
teresting fact that in certain Prawns and Stomapoda (as probably 
Squilla mantis) similar conditions occur. The metamorphosis of the 
former commences sometimes (as in the Cirripeds) with monoculoid 
forms, and passes through very peculiar Zoéoid and Mysis-like states, 
sometimes with Zoéa forms which in structure and mode of movement 
resemble those of Hermit Crabs, whilst in others we can hardly say 
that there is any metamorphosis. Dr. Miller has, however, described 
and figured a little animal which he considers the Zoéa of a Stomapod, 
of glassy transparency, in which the segments exist in almost the same 
number as in the mature Stomapods, the sixth and seventh abdominal 
segments only being not yet distinct from each other. As in the Zoée 
of the Crabs and Porcellane, the appendages of the sixth hinder 
thoracic segments, and the lateral lamine of the caudal fin were also 
as yet entirely deficient. They possess only a median eye. 
So little is known of the habits and modes of life of marine animals, 
that we cannot but feel much interested and deeply indebted for care- 
fully observed facts in this department. Such are those of the curious 
