STRUCTURE OF THE BRAIN OF THE SESSILE-EYED CRUSTACEA. 9 



regious. The foregoing observations on the brain and eyes of the common Asellus of onr brooks and 

 ponds were made to afford a basis of comparison with the similar parts in the eyeless form. 



Caicidotiea in its external shape is seen to be a depaui^erate Asellus, with the body, however, 

 much longer and slenderer than in the eyed form, and with slenderer appendages. It is not usually 

 totally eyeless. In a number of specimens from a well at Normal, III., kindly sent us by Mr. 

 S. A. Foi'bes, a minute black speck is seen on each side of the head in the positions of the eyes of 

 Asellus, just above the posterior end of the base of the mandibles. In some specimens these black 

 dots are not to be seen; in others thej' are visible, but fainter than in others. In twelve specimens 

 which I collected in Shaler's Brook in Mammoth Cave I could detect no traces of eyes, and infer 

 that most, if not all, the Mammoth Cave specimens are totally eyeless. It thus appears that dififer- 

 eut individuals have eyes either quite obsolete, if living in caves in total darkness, or, if living in 

 wells, with eyes in different degrees of development up to a certain stage — that represented by 

 black dots — which, however, are so easily overlooked, that we confess, after handling dozens of 

 specimens, we did not suspect that the rudimentary eyes existed, until our attention was called to 

 them by Dr. C. O. Whitman when he sent the slides. The European Gwcidotcca forelii is also said 

 to be blind. The si^ecimens we received through the kindness of Professor Forel, which were, 

 unfortunately, dried and spoiled, seemed to be entirely eyeless, though special search was not 

 made for the eye-specks. 



It will be seen that the eyeless Ciecidot?ea differs from Asellus as regards its brain and organs 

 of sight, in the complete loss of the optic ganglion, the optic nerve, and the almost and sometimes 

 quite total loss of the pigment-cells and lenses. 



After a pretty careful study of numerous vertical sections of the brain of Cwcidofwa sfygia as 

 conipai'ed with that of Asellus communis we do not see that there are any essential differences, ex- 

 cept iu the absence of the optic ganglia and nerves. The proportions of the procerebral lobes, of 

 the ganglion cells, their number and distribution, the size of the transverse and longitudinal com- 

 missures are the same. The head and brain as represented is smaller than in Asellus, the form 

 itself being considerablj' smaller. 



Fig. 25 represents a section through the middle of the procerebral lobes, which may be com- 

 pared with that of Asellus, Fig. 4. Another section a little posterior is represented by Fig. 26. 

 Fig. 27 is an enlarged view of a section still further back, which shows that there is little, if any, 

 difference between the brain at this point and that of Asellus I'epresented by Fig. 3. In this sec- 

 tion it is easy to see that the ganglion cells on each side of the procerebral lobes send fibers 

 directly through the myeloid mass to form the transverse commissures. The section at this point 

 does not show the fibers arising from the frouto-dorsal group of ganglion cells; but traces of them 

 are seen in Fig. 28, which represents a section corresponding to that indicated by Fig. 3. 



Careful examination of the sections passing behind the procerebral lobes and (esophageal com- 

 missures failed to show any traces of the optic ganglion of either division, or of the ganglion cells 

 and myeloid substance composing it. Every part connected with the optic ganglia seems to be 

 totally abolished. The same may be said of the optic nerve throughout its length. The amount 

 of time spent in examining the numerous well cut, thin, and beautifully mounted sections made by 

 Dr. Whitman, or under his direction, enables us to affirm positively that the entire nervous portion 

 of the optical organs are wanting. And we are glad to add that Dr. Whitman also observed to us 

 the absence of the optic nerves. 



With the eye itself it is difl'erent. The modification resulting from a life in total darkness has 

 left traces of the eye, telling the story of degeneration and loss of the organs of sight, until but 

 the merest rudiments of the eye remain as land marks pointing to the downward path iu deg- 

 radation and ruin taken by the organs of vision as the result of a transfer to a life in total or par- 

 tial darkness, as the case may have been, in the well-inhabiting or cave-dwelling individuals. 



Fig. 29 represents a section through the head of Gcccidotwa stygia behind the procerebral lobes 

 and oesophageal commissures, showing the absence of any traces of the optic ganglia or optic nerves, 

 but indicating the rudiments of the eye, showing that the pigment mass of the retina and the 

 lenses exist in a very rudimentary condition, while the optic nerve and ganglion are entirely 

 aborted. 



2 CRU 



