CRUSTACEA. 



763 



as finally to constitute a single continuous 

 mass only. The same thing happens in re- 

 gard to the fifth and sixth, which soon form no 

 more than a single ganglion. As to the other 

 pairs they always remain completely distinct, 

 and some way parted from one another. 



Thus the study of the gradual evolution of the 

 nervous system in the Astacus fluviatilis, al- 

 though by no means belonging to the type in 

 which this system is most completely developed, 

 presents us with three distinct and successive 

 facts, which we shall find reproduced in the 

 most perfect manner in the natural series of 

 genera, and which will put us into a position 

 to give a satisfactory explanation of those very 

 striking variations in the organization which we 

 shall encounter. 



These are, in the first place, the isolated for- 

 mation of the nervous centres, independently 

 one of another. We now acknowledge this 

 independence of the several organs at the 

 moment of their appearance, and their ulterior 

 conjunction is one of the most interesting and 

 important facts with which modern science has 

 been enriched ; it constitutes the law of centri- 

 petal development, as it has been established by 

 M. Serres. 



In the second place a tendency to conjunc- 

 tion by a motion transversely. 



Lastly, a second motion in the line of the 

 axis of the body, the effect of which is the 

 concentration definitively of a greater or smaller 

 number of nervous centres primarily indepen- 

 dent of one another. 



The Talitrus exhibits in the 

 most striking manner the first of 

 the three dispositions which we 

 have mentioned from the mo- 

 ment at which the nervous sys- 

 tem appears. In this genus, in 

 fact, we perceive on either side 

 of the median line a ganglionic 

 chain, formed by the conjunc- 

 tion of the nervous centres, 

 extremely simple in their struc- 

 ture, and flattened and some- 

 what lozenge-shaped in their 

 outline.* There are thirteen 

 pairs thus constituted, corres- 

 ponding to the thirteen seg- 

 ments which enter into the com- 

 position of the whole body. The 

 two nuclei of each pair com- 

 municate together, in the same 

 manner as each pair is con- 

 nected with that which succeeds, 

 and with that which precedes it, 

 by means of medullary cords in 

 the first instance, and longitu- 55JjdE5 

 dinal cords in the second In hali '. 



all essential particulars each pair glia : ^ me _ 

 is a counterpart of any and dullary cords 

 every other pair, without even uniting the first 

 excepting the cephalic ganglion, and second pair 

 and it is with difficulty that the of S anglia ' 



* Vide Recherches Anatomiques snr le Systeme 

 Nerveux des Crustaces, par M M. Audouin et Milne 

 Edwards, Annales des Sciences Naturelles, torn. 

 14. 



Fig. 391. 



thoracic pairs are seen to be in a slight degree 

 larger than the others. At a somewhat greater 

 distance forward from the oesophagus, too, than 

 usual, we observe the cephalic ganglion, which 

 sends branches to the antennas and eyes, and 

 the nervous cords by means of which it commu- 

 nicates with the ganglions of the first thoracic 

 rings. These cords, having the oesophagus inter- 

 posed between them, are held a little farther apart 

 than the other branches, which establish com- 

 munications between the different succeeding 

 pairs of ganglions in the longitudinal direction. 



Already in the Oniscus asellus* and in the Cy- 

 amus ceti-\- we find the ganglionic cord, double 

 in its middle portions, simplified at its opposite 

 extremities in such wise that the ganglions of 

 the first and of the last pairs are single. This 

 commencement of approximation coincides in 

 other respects with an incipient approximation 

 in the longitudinal direction, for, to the four- 

 teen segments of which the whole body consists, 

 we find no more than ten pairs of ganglions 

 apportioned. 



This tendency to centralization is still more 

 conspicuous in the Phyllosoma.J Here we 

 discover the two cephalic nuclei united by their 

 internal angle, without, however, their state of 

 doubleness being thereby obscured. It is the 

 same with the first pair of thoracic ganglions, 

 from which they are separated by the whole 

 length of the great oval lamina which supports 

 the cephalic appendages and is traversed 

 lengthwise by the nervous filaments which 

 embrace the oesophagus. The ganglions of the 

 second pair, although rudimentary, are still 

 united immediately, as are those of the third 

 pair also. Those of the six suc- 

 ceeding pairs, on the contrary, Fig. 392. 

 only communicate by means of 

 a transverse but thick and - short 

 commissure, so that it gives to 

 the connexion established be- 

 tween the nuclei of the several 

 pairs, the appearance of a more 

 immediate conjunction than ac- 

 tually exists. To conclude, the 

 abdominal ganglions are perfectly 

 distinct, and those of the several 

 pairs are only connected by 

 means of extremely slender fila- 

 ments. 



In the Cymothoa the union of 

 the medullary nuclei in the trans- 

 verse direction is complete, and 

 all we perceive is a single series 

 extended along the median line 

 through the whole length of the 

 body. This is similar to the 

 nervous system of the Talitrus 

 conjoined longitudinally; with 

 this difference, that the longitudinal filaments 

 uniting the ganglion have continued distinct, 

 as if to testify, by their doubleness, to the 

 mode of formation of the single ganglionic cord. 



* Cuvier. Lecons d'Anatomie comparee, t. ii. 

 p. 314. 



t Treviranus, Vermischte Schriften anatomischer 

 und physiologischer inhalts, Band 2. Heft I. 



J Audouin et Edwards, loc. cit. 



3 D 2 



