§^ 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 abdon)inal 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, Unerkanute Struct. &c. p. 56, 

 Tab. VI. fig. 3-5 (Homarus marinus, Astacus 

 Jluviatilis and PaLaemon squilla). The vari- 

 cose enlargements are represented too regular in 

 some of these figures. See, also, Hannover, Ee- 

 cherch. kc. p. 6S, Tab. VI. fig. 76, c. e. 



S Hannover, loc. cit. p. 67, fig. 75, 76-a. (^Asta- 

 cus Jluviatilis), and Valentin, in the Nov. Act. 

 Acad. Nat. Cur. XVIII. p. 210, Tab. IX. fig. 73- 

 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 New- 

 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 Philjs. Transact. 1843, p. 243, PI. XI., or in 

 abstract in Froriep^x neue Notiz. XXVIII. p. 177, 

 or in the Ann. d. Sc. Nat. 1. 1844, p. 58, or Annals of 

 Nat. Hist. XII. ]). 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 

 pass from one side of the ganglion to the other ; and 



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 

 oi Jibres of reinforcement. Eacii peripheric 

 nerve given off from the abdominal cord, contains 

 fibres fr(jm 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 sjTn- 

 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 deviated 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 

 durmg 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. 



- For the nervous system of the macrourous 

 Crustacea, see Audouin and Milne Edwards, loc. 

 cit. (Homarus, Palaemon, and Palinurus) ; 

 Suckow, loc. cit. p. 61, Taf. XI. fig. 7 (Astacus); 

 Brandt, Medizin. Zool. I. p. 64, Taf. IX. fig. 1, 

 and especially Newport, Philos. Trans. 1834, p. 

 406, PI. XVII. fig. 40-42 (Homarus). 



* [ § 271, note 1.] See, also, for histological details liker's Zeitsch. Ft. p. 328, III. p. 291 {Argulua, 

 on this system, Ley dig ^ loc. cit. Siebold and Kiil- Artemia, Branchipus). — Ed. 



27* 



