ON THE STRL CTUHE OF THE BRAJN OF THE SESSILEEYED CRUSTACEA. 



READ AT WASHIXUTON, APRIL 14, 1864. 



U,V A. S. Packaud. 



The following- descriptions aud notes liave grown out of :iu attempt to compare the nervous 

 system, particularly the brain and other ganglia of the head, of the eyeless species of cave inhabiting 

 Arthropods with their oat of-door allies. We have begun with the structure and morphology of 

 the brain of Aselliis communis Say as a standard of comparison with that of the blind Asellid, 

 Gecidotcva sijiqia Pack., which is so common in the brooks of Mammoth and other caves and in the 

 wells of Southern Indiana and Illinois. Studies of this nature are, it seems to us, well calculated 

 to throw light on the origin of the cave forms, and to show what great modiftcatious have been 

 produced iu these organisms by a radical change in their surroundings ; consisting, as it does, nminly 

 in the absence of light, and perhaps of the usual food, or at least the usual amount of food. 



It is plain enough that the species of Gecidotsea are simply eyeless, slender, depauperated 

 Aselli, which have originated from some one of our out-of-door species within a comparatively 

 recent time, at least since the river-terrace epoch of the Quaternary Period. The facts bearing 

 upon the general relations of the blind to the eyed Asellida^, and a discussion of the change iu 

 form of the body and its appendages, and of the causes of the transformation of the species and 

 genus, are reserved for another occasion. 



My present purpose is simjily to describe aud depict the brain and other nerve-centers of the 

 head oi AsdJus communis Say aud Gccidotaa styyia Pack. 



I. The bkain of Asellus communis. 



The nervous system of tlie European Asellus ((quaticui Linn, has been referred to by Leydig 

 aud also by Sars, who published a tigure of the nervous system as a whole. Leydig's " Vom Bau 

 des thierischcii Korijcrs^'' gives a careful and comprehensive general account of the nervous system of 

 Arthropods, the most complete and authoritative, up to ISOA, we jiossess, supplemented as it is by 

 his excellent Tn/eln ron veigleicluiidcn Anatomic, published iu the same year (18(Ji). According to 

 Leydig, in the Isopoda (Oniscus, Porcellio) the optic lobes are very large and overlie the cerebral 

 lobes. 



In Asellus aquaticus the abundant fat body around the ventral cord belongs to the blood sinus 

 which envelops the nervous cord. Of this form Leydig has little to say, remarking that he did not 

 examine the* entire ventral cord, but only sections, which agree iu appearance with those of the 

 land wood-lice. 



Sar's tigure of the brain of Asellus aquaticus is drawn on a small scale, is rather indiffereut, aud 

 does not show more than the cerebral lobes and optic nerves. He evidently did not i)erceive the 

 other ganglia. 



Leydig's valuable figures of the biaiu of Oniscus murarius show that he did not study the 

 nervous centers of the head by means of longitudinal sections, aud that he simply dissected the 

 brain from above, a dorsal view showing tiie large optic lobes to be mostly above and iu front of 

 the smaller cerebral lobes, while the ganglion, e, in his tigure 8 (Taf. VI), which he denominates 

 nehcnlappen, is probably one of the antennal ganglia. The other ganglia of the head he does not 

 represent, nor speak of in his Vergieichende Anatomic. 



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