CRUSTACEA. 



767 



a series of independent centres, and we have 

 seen these becoming successively conjoined in 

 a greater and greater degree, as if in obedience 

 to a law of attraction, whose tendency was to 

 collect these various nuclei from every part of 

 the body towards a common centre. This dis- 

 position to centralization has, in its turn, given 

 a satisfactory explanation of the most remark- 

 able differences observed in the disposition of 

 the ganglions and of the nervous cords 

 among the different types of the class, however 

 dissimilar these may be one from another. 

 We may, therefore, here conclude, as has been 

 done already in my work especially devoted to 

 this subject, that the nervous system of the 

 Crustacea consists uniformly of medullary 

 nuclei (ganglions), the normal number of' 

 which is the same as that of the. members or 

 rings of the body, and that all the modifica- 

 tions encountered, whether at different periods 

 of the incubation, or in different species of the 

 series, depend especially on the approximation, 

 more or less complete, of these nuclei, (an ap- 

 proximation which takes place from the sides 

 towards the median line as well as in the longi- 

 tudinal direction,) and to an arrest of develop- 

 ment occurring in a variable number of the 

 nuclei. 



In a paper upon the nervous system of the 

 Lobster recently published,* Mr. Newport 

 mentions an interesting fact hitherto overlooked 

 by anatomists. He found that the double 

 ganglionic chain of this Crustacean is composed 

 of two orders of fibres, forming distinct and 

 superposed fasciculi or columns, which the 

 author designates columns of sensation and of 

 motion, following the analogy which he be- 

 lieved he had traced between these fasciculi 

 and the anterior and posterior columns of the 

 spinal cord of the higher animals. The fas- 

 ciculi here indicated are but indistinct in the 

 interganglionic cords, but become extremely 

 apparent in the ganglions themselves, for these 

 enlargements belong exclusively to the inferior 

 or sensitive fasciculi, and the superior or motor 

 fasciculi pass over their dorsal surface without 

 penetrating their substance at all. 



Before going on to the study of those organs 

 the object of which is the application, if we 

 may be allowed the expression, of the nervous 

 system to the perception of the existence of 

 outward objects, and of those in which the 

 reaction designated volition is immediately 

 effected, that is to say, the organs of the senses 

 and the muscles, it may be as well to say a 

 word upon the general functions of the nervous 

 system itself in its different parts. The 

 experiments made by M. Audouin and me, 

 with a view to solve the principal problems 

 which may be proposed on this subject, have 

 confirmed the inductions to which we had been 

 led by views arrived at a priori wholly from 

 anatomical researches, of which the preceding 

 may be regarded as the summary. Tims : 



* On the Nervous System of the Sphinx ligustri, 

 &c. by G. Newport, Philos. Transact. 1834, pt. ii. 

 p. 406. 



Istly, The nervous is the system which en- 

 tirely presides over the sensations and motions. 

 2dly, The nervous cords are merely the 

 organs of transmission of the sensations and 

 of volition, and it is in the ganglions that the 

 power of perceiving the former and of pro- 

 ducing the latter resides. Every organ sepa- 

 rated from its nervous centre speedily loses all 

 motion and sensation. 



3dly, The whole of the ganglions have 

 analogous properties : the faculty of determin- 

 ing motions and of receiving sensations exists in 

 each of these organs; and the action of each 

 is by so much the more independent as its 

 development is more isolated. When the 

 ganglionic chain is nearly uniform through its 

 whole length, it may be divided without the 

 action of the apparatus being destroyed in 

 either portion thus isolated, always under- 

 stood, that both are of considerable size; 

 because when a very small portion only is 

 isolated from the rest of the system, this 

 appears too weak, as it were, to continue its 

 functions, so that sensibility and contractility 

 are alike speedily lost. But when one portion 

 of the ganglionic chain has attained a develop- 

 ment very superior to that of the rest, its 

 action becomes essential to the integrity of the 

 functions of the whole. 



It must not be imagined, however, from 

 this that sensibility and the faculty of exciting 

 muscular contractions are ever completely con- 

 centrated in the cephalic ganglions, and it 

 seems to us calculated to convey a very 

 inaccurate idea of the nature and functions of 

 these ganglions to speak of them under the 

 name of brain, as the generality of writers have 

 been led to do, seduced by certain inconclu- 

 sive analogies in point of form and position. 

 It is nevertheless to be remarked that in these 

 animals an obscure tendency to the centra- 

 lization of the nervous functions is observable 

 in the anterior portion of the ganglionic chain ; 

 because if in the Lobster, for instance, it he 

 divided into two portions, as nearly equal as 

 possible, by severing the cords of communica- 

 tion between the ganglions belonging to the 

 first and second thoracic rings, sensibility, and 

 especially mobility, are much more quickly 

 lost in the posterior than in the anterior half; 

 and this disproportion is by so much the more 

 manifest as the division is performed more 

 posteriorly; still there is a great interval 

 between this first indication and the concen- 

 tration of the faculties of perception and of 

 will in a single organ the brain, of which 

 every other portion of the nervous system then 

 becomes a mere dependency. 



B. Organs of the senses. Do the five 

 senses exist, and to what degree of development 

 have they attained in the Crustacea? Such 

 is the question we have now to consider, and 

 which we shall sometimes find ourselves in a 

 condition to answer from the simple inspection 

 of the various organs of special application. 



Thus we discover almost at once that the 

 sense of general touch is obtuse, and can 

 convey to the animal no other but confu-.-d 



