816 THE CRUSTACEA. $ 270. 
CHAPTER III. 
NERVOUS SYSTEM. 
§ 270. 
The Nervous system of the Crustacea, is developed in different de- 
grees according to the various orders. 
Its central mass consists of an abdominal cord, connecting, usually, with. 
the cerebral ganglia by an cesophageal ring. With the long-bodied species, 
this abdominal cord is composed of numerous ganglia, arranged in succes- 
sive pairs from before backwards, and connected together by longitudinal 
commissures. But when the cutaneous skeleton is shortened by a dimi- 
nution or a fusion of the segments, the ganglionic chain is lessened in a like 
manner by a coalescence or a disappearance of several of its ganglia. 
With the Macrura, the Stomapoda, the Amphipoda, and Isopoda, the ab- 
dominal cord consists of ten to thirteen pairs of unequal ganglia, situated, 
usually, on the median line, and shielded by septa given off from the inter- 
nal surface of the thoracic and abdominal segments of the skeleton. 
The size of these ganglia is in direct ratio with the development of the 
segments and their appendages, to which they belong. Those of the 
thorax, — the anterior abdominal ones, as well as the last caudal one, are 
consequently very large, for they send filaments to the various chelate, 
prehensile, ambulatory, and natatory appendages, and to the caudal lam- 
ellae, which are usually highly developed. With the Myriapoda, the ab- 
dominal cord is remarkable for the great number of its ganglia, which are of 
equal size. Quite often, the ganglia of the same pair are fused into a 
single mass ; in which case, the two interganglionic commissures are more or 
less approximated or even blended together. With some species, a portion 
of the abdominal ganglia are so closely approximated, successively, that the 
interganglionic commissures are wholly wanting. With the Brachyura, the 
whole abdominal cord is concentrated into a large central mass. 
The peripheric nerves arise from the ganglia, rarely from the intergan- 
glionic commissures. The cerebral mass, which is situated above or in 
front of the cesophagus, is composed of ‘a pair of considerable ganglia, 
more or less fused together. The nerves sent off from these, go princi- 
pally to the organs of sense; and in the inferior Crustacea, where these last’ 
are wanting, the cerebral mass is absent also. In such case, there are 
usually wanting likewise the two cerebral commissures, which are given 
off from the anterior thoracic ganglion, and surround the cesophagus.* 
1 Audouin and Milne Edwards (Amn. 4. 8c. 
Nat. XIV. 1828, p. 77, Pl. Il.-VI.) have given a 
general review of the disposition of the nervous sys- 
tem in the different orders of Crustacea ; see also 
* [End of § 270.) Itis regretted that no example, 
illustrative of this last statement, is given, for 
certainly none is now recollected where the grand 
typical structure is not present, —in other words, 
where the cesophagus does not pierce the cerebral 
nervous system.at some point. In many of the 
Miine Edwards, Hist. Nat. d. Crustac. I. p. 126, 
Pl. XI., and his article Crustacea in the Cyclop- 
of Anat. loc. cit. p. 762. 
inferior Crustacea, such as Caligus, and some of 
not all of the Cyclops tribe, the cephalic, thoracic, 
and abdominal ganglia, are fused into a single 
mass through the anterior part of which the ceesopha- 
gus passes ; see Dana, loc. cit. Caligus. Amer. Jour. 
Sc. XXXIV. p. 250.— Ep. 
