CRUSTACEA. 337 



which lias yet occurred to our notice have we been able strictly to 

 point out the accuracy of such a view of the subject. The two 

 lateral masses of the supra-cesophageal ganglion are found united 

 into one brain in the humblest forms of annulose animals, and even 

 in the ganglia forming the ventral series, although we might pre- 

 sume each to be composed of two symmetrical halves, the divisions 

 are most frequently so intimately blended, that their distinctness 

 is not susceptible of anatomical demonstration. In some of the 

 Crustacea, however, among those species which have the segments 

 of their external skeleton most perfectly separate and distinct, the 

 nervous system is found to present itself in such a condition that 

 the division into lateral halves is perfectly evident ; and from this 

 condition their progressive coalescence may be traced step by 

 step until we arrive at a state of concentration as remarkable 

 as that already noticed in the most elevated of the Arachnidans. 

 It is to Milne Edwards and Audouin that we are indebted for 

 the interesting particulars connected with this part of our sub- 

 ject ; and the results of their investigations are of such great 

 physiological importance,* that the following condensed ac- 

 count of their labours cannot be omitted in this place. In 

 Talitrus every pair of ganglia consists of two separate nuclei of 

 nervous substance, united by a transverse band so disposed as to 

 bring them into communication with each other, while an anterior 

 and posterior nervous filament derived from each unites it with the 

 preceding and following ganglia of the same side of the body ; 

 even the encephalic mass is composed of two lateral portions united 

 by a cord passing between them : all these pairs of ganglia, thir- 

 teen in number, corresponding with the number of the segments 

 of the body, are exact counterparts of each other both in size and 

 figure, so that none seems to preponderate in energy over the rest ; 

 but the anterior or encephalic pair alone communicates with the 

 eyes and antennae, the only organs of the senses as yet discernible. 

 In Oniscus Asellus a concentration of the elements composing 

 the nervous system above described is discernible, and this is found 

 to be indicated by incipient approximation, which takes place in 

 two directions, one longitudinal, the other acting transversely. In 

 the first place, the entire number of pairs of ganglia is reduced to 

 ten, three pairs having become obliterated by coalescence ; and, 

 moreover, while the central portions still consist of two lateral 



* Recherches Anatomiques sur le Systeme Nerveux des Crustacea. Annales des 

 Sciences Nat. torn. xiv. 



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