$$ 271, 272. THE CRUSTACEA. 817 
§ 271. 
The intimate structure of the nervous system in many of the orders of 
Crustacea, can be made out without difficulty, by dissection and the micro- 
scope; for its elements are not as liable to change as in the other classes 
of the Invertebrata already described. 
In many species, there may be observed in the nerves surrounded by a. 
delicate fibrous neurilemma, the primitive nerve-fibres so large that their 
double contour is easily seen; but these gradually assume a varicose as- 
pect.” In the ganglia, the ganglionic globules may be easily seen, as 
very large, round, and sometimes pyriform cells, having each a dispropor- 
tionately large nucleolated nucleus.® As to the course and arrangement 
of the nerve-fibres in the interior of the abdominal ganglia, two kinds of 
these fibres may sometimes be distinguished: the first pass uninterruptedly 
through all the ganglia successively, and thus contribute to the formation 
of the longitudinal commissures; but the second pass round among the 
ganglionic globules, and emerge laterally from the ganglion to form the 
peripheric nerve. 
« 
§ 272. 
From a more particular examination of the arrangement of the nervous 
system in the different orders of Crustacea, the following remarkable facts 
have been noticed.” 
With the Macrura, where this system is most highly developed,” the 
abdominal cord is composed of twelve pairs of ganglia, generally blended 
1 See Helmholtz, De fabric. syst. nerv. everte- 
brat. loc. cit. p. 17.* 
2 Ehrenberg, Unerkannte Struct. &c. p. 56, 
Tab. VI. fig. 3-5 (Homarus marinus, Astacus 
JSluviatilis and Palaemon squilla). The vari- 
cose enlargements are represented too regular in 
some of these figures. See, also, Hannover, Re- 
" cherch, &c. p. 68, Tab. VI. fig. 76, c. e. 
8 Hannover, loc. cit. p. 67, fig. 75, 76 a. (Asta- 
cus fluviatilis), and Valentin, in the Nov. Act. 
Acad. Nat. Cur. XVIII. p. 210, Tab. IX. fig. 72 
85. This last author declares that he has observed, 
in the abdominal ganglia of the common crawfish, 
the ganglionic globules divided symmetrically into 
two groups, right and left; and in the caudal gan- 
glia of the same species, that he has seen two double 
groups, two anterior and two posterior. 
4 Valentin (loc. cit. p. 211) has seen these two 
kinds of primitive fibres in the abdominal cord of 
the common crawfish. We are indebted to Vew- 
port for very complete observations on the disposi- 
tion of the nervous fibres in the abdominal cord of 
Myriapoda, and illustrated by numerous figures ; 
see Philos. Transact. 1843, p. 243, Pl. XI., or in 
abstract in Froriep’s neue Notiz. XXVIII. p.177, 
or in the Amn. d. Sc. Nat. I. 1844, p. 58, or Annals of 
Nat. Hist. XII. p. 223. According to this observer, 
four fasciculi of primitive nerve-fibres may be ob- 
‘served in the ventral cord of the Myriapoda. An 
upper and a lower, extending longitudinally, con- 
tain the one, motor, and the other, sensitive fibres. 
A third is composed of transverse fibres which 
pasa from one side of the ganglion to the other ; and 
* [ § 271, note 1.] See, also, for histological details 
on this system, Leydig, loc. cit. Siebold and Kél- 
27* 
the fourth extends from one ganglion to the next 
succeeding, by the side of the longitudinal commis- 
sures. To these last, Newport has given the name 
of fibres of reinforcement. Each peripheric 
nerve given off from the abdominal cord, contains 
fibres from all of these four fasciculi. The associate 
and reflex motions between the feet of the same 
pair, are due to the transverse fibres, and the sym- 
pathy between the posterior and anterior feet is 
referable to the fibres of reinforcement. 
1 If, in proceeding from the higher to the lower 
species in the description of the nervous system, I 
have deyiated from the plan hitherto pursued, it is 
because, with the Crustacea, this system, notwith- 
standing the various forms of the body, is found 
upon one and the same type, which is not true in 
any of the preceding classes, as, for instance, in 
the Acephala, This type is especially apparent 
during the young age of these animals, and does 
not change except from their ulterior metamor- 
phosis, when, often some portions of the nervous 
system disappear ; on this account, this last will be 
best understood when studied in its primitive state, 
or from the more perfect forms it presents in the 
higher Crustacea. 
2 For the nervous system of the macrourous 
Crustacea, see Audouin and Milne Edwards, loc. 
cit. (Homarus, Palaemon, and Loe 
Suckow, loc. cit. p. 61, Taf. XI. fig. 7 (Astacus) ; 
Brandt, Medizin. Zoot. I. p. 64, Taf. IX. fig. 1, 
and especially Newport, Philos. Trans. 1834, p. 
406, Pl. XVII. fig. 40-42 (Homarus). 
liker’s Zeitsch. II. p. 328, III. p. 291 (Argulus, 
Artemia, Branchipus). —Ep. 
