104 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



The first suhcesoplMf/cdl or mandibular ganglion (Figs. 1, 6, 7, 22, 23, md. g.). — This is rather 

 hirger than eitlier of the anteniuil ganglia, as its rehitious to the braiu are well seen in the sections 

 represented by Fig. 0. By reference to the sections represented by Figs. 5 and 6, it is clearly seen to 

 lie directly under the aiitenual ganglia, and to be separated from the brain proper by the short 

 (esophageal commissures. It is therefore the first subnesophageal ganglion, givin g oft' but a single 

 pair of nerves, those supplying the large tripartite mandibles. 



The ganglion lies in front of the main longitudinal commissure, and in position in front of 

 the lower side of the stomach, being situated in an inclined plane, nearer vertical than horizontal 

 The sections represented by Figs. 22 and 23 pass through a portion of it, and in them is well seen, 

 the mode of origin of the large mandibular nerves. 



The first and second maxillary ganglia. — These are situated widely apart, neither coalescing 

 with the other ganglia in front or behind. The first maxillary ganglion (Figs. 1, 8, 22, 23, mx.g.) 

 is situated nearer the mandibular than the second maxillary ganglion, as seen in Figs. 1, 22, and 23. 

 It lies in an inclined plane, and is much smaller than any of the other postcesophageal ganglia, 

 as it innervates smaller appendages. 



The second maxillary ganglion (Figs. 1, 8, 22, 23, jhj;^ (/.) is situated next to the maxillipedal 

 ganglion, and like that lies in a horizontal position. It is of nearly the same size but a little 

 smaller than the ganglion next behind it, and the commissures connecting it with the maxillipedal 

 ganglion are very short. 



The maxillipedal ganglion (Figs. 1, 8, 22, 23 mxp. g.) is a little larger than its near neighbor, the 

 second maxillary ganglion, inasmuch as it innervates the large maxillipedes. 



At some distance behind this ganglion and situated in the first thoracic segment is the first 

 thoracic ganglion supplying the nerves to the first pair of feet. It is a little larger than the max- 

 illipedal ganglion. 



The main longitudinal commissures (Figs. 1, 22, 23) pass over the ganglia, and are united in the 

 head, except at two points indicated by the clear spaces in the figure, behind which point we have 

 not traced it. Sars, however, represents the main longitudinal commissure behind the head as 

 double. 



In the section represented by Figs. 22 and 23 the limits of the mandibular and first maxillary 

 ganglia are not definite, and they are seen to be connected by a bridge or tract of myeloid sub- 

 stance. Towards the second maxillary ganglion the fibers in the section are fewer and lower 

 together, and are seen in some cases to enter the myeloid substance, but in others to pass over it. 

 The ganglion cells of the maxillipedal ganglion are more numerous than those about the myeloid 

 mass of the second maxillary ganglion 



From the foregoing facts it will be seen that the brain of the AselUdw is composed of four 

 precBSophageal p lirs of ganglia, situated at greater or less distance apart from each other, being 

 a very loosely constructed syncerebrum compared with that of such Decapods as have been thus 

 far examined. The mouth-parts in the Asellidw, if not all Isopoda, are not innervated from a single 

 subtesophageal ganglion, but each appendage, beginning with the mandibles, is supplied by a nerve 

 arising from a separate ganglion. Thus there are eight ganglia of the first order in the head of 

 these Isopods, our observations not referring to any secondary ganglia, which may or may not exist 

 in connection with the brain or sympathetic nervous system. It will be remembered that in the 

 Decapods, the lobster for example, the brain innervates the eyes and an tenure, while the only other 

 ganglion in the head is the suboesophageal, from which the mouth appendages are all innervated ; 

 thus there are but two nerve-centers in the head of adult Decapods; the subtesophageal ganglia 

 being concentrated probably during embryonic or larval life. 



II. The brain of the eyeless form C^cidot^a. 



It is a matter of great interest to know just what, if any, changes take place in the braiu or 

 nerve-centers of the head of the eyeless forms related to Asellus; whether the modification is 

 confined to the external parts of the eye, or to the optic lobes and nerves alone. 



It is well known that a blind Asellus-like form is abundant in the brooks and pools of Mam- 

 moth and other caves in Kentucky and Indiana, as well as in the wells of the cavernous and adjacent 



