ox 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 

 an'ived 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 



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