204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



trance is a little to one side of the median plane of the brain, opposite the 

 posterior end of the globulus (Plate 9, Fig. 41). Its fibres run backward 

 and dorsad, just lateral to those of the autenuular nerve, and end in a 

 neuropil directly anterior and median to that of the second antenna 

 (Fig. 41, n. ot.). The individual fibres end by branching into fine 

 fibrillations, which could be traced only a short distance through the 

 diffusely stained mass of fibrillar tissue about them. 



d. Histology of the Nerve Elements. The sensory nerve fibres of 

 Cambarus are relatively smaller than those of Pal^monetes. Imme- 

 diately after leaving the ganglion cell each measures about 3|U in 

 diameter, but becomes smaller as it runs distally, until near the point 

 of ending, where it again enlai'ges to its original size. In well differen- 

 tiated methylen-blue stains, fibrillar structure is clearly brought out. 

 Longitudinal sections, and whole preparations of continuous fibres, show 

 fibrillations similar to those figured for Palaemonetes. 



The sensory nerve cells are relatively large ; they measure from 15 to 

 18 jU in diameter, and being bipolar are spindle-like in form. Their 

 nuclei are spherical and from 10jutol2^ in diameter. The cytoplasm 

 of the cell never shows any evidence of fibrillations, but in methylen-blue 

 impregnations there is a faintly staining zone directly about the nucleus ; 

 the remainder of the cytoplasm takes on a deep blue color. This dif- 

 ference in staining qualities may be due to the unequal distribution of 

 chromatic substance in the cytoplasm. 



The myelin sheath, so characteristic for the nerve fibres of Palaemo- 

 netes and Crangon, is not found in the nerve elements of the crayfish. 



3, The development of the otocyst was not studied in Cambarus. Ac- 

 cording to Reichenbach i^^^^ it is completely formed before the youug 

 animal leaves the egg. 



IV. Carcinus MiENAS Leach. ( Greeii crab. ) 



We now come to the second type of otocyst, which is found in all 

 brachyuran Crustacea ; it is closed, and without otoliths. Mistaken by 

 Bate ('58) for an olfactory organ, and figured by him in the larval 

 stages of the crab, it has been described carefully in Carcinas mannas 

 by Ilcnsen ('63) alone. His account, although fairly accurate, is in- 

 fluenced by his seeing a fancied resemblance between the otocyst and the 

 vertebrate ear ; the figures he gives of different parts of the sac dissected 

 out leave one somewhat in the dark as to the relative positions of the 

 structures described. 



