ON CRUSTACEA. 
243 
It forms no part of our plan to describe minutely the 
muscles of the Crustacea. The reader who is desirous of 
further information on this subject, must be referred to special 
works on the anatomy of these animals, and more particularly 
to the Comparative Anatomy of Cuvier. We shall simply 
confine ourselves to stating that those of the feet of the 
brachyuri are very powerful, and placed in sorts of lodges, 
which are formed under the testa by certain vertical solid 
partitions which separate the different pieces of the breast- 
plate, that those of the tail of the macrourous decapods, when 
arrived at the maximum of development, are very complicated, 
and form a dorsal mass, which is rather thin, and a ventral 
mass very thick, both composed of three orders of well-marked 
fibres, finally, that in certain small entomostraca, particular 
muscles which do not exist in others are destined to fix the 
animal to its shell, and to enable it to open or shut the valves 
of the latter, according to inclination. 
As to the function of sensibility , the Crustacea have a 
nervous system very similar to that of insects and arachnida. 
It principally consists in a brain placed in front of, and above 
the intestinal tube, and in an elongated medulla, composed of 
a double knotty cord, placed at the lower face of the body, 
sometimes, as in the macrourous decapods, extending through 
the entire length of the body ; and sometimes, as in the 
brachyuri, forming towards the middle of its lower face, a 
medullary circle from which the nerves issue in radiations. 
“ The brain,” says M. Cuvier, in his Comparative Anatomy, 
“ in the animals of these two families, is situated at the an- 
terior extremity of the body. Its mass is more broad than 
long, and its superior face is divided into two rounded lobes. 
The middle lobes furnish, each from the anterior edge, an optic 
nerve, and which proceeds directly into the peduncle of the 
eye. This nerve is divided into a multitude of threads, each 
of which is carried to one of the particular eyes which form 
R 2 
