204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
trance isa 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 antennular 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 Palemonetes. Imme- 
diately after leaving the ganglion cell each measures about 3yu in 
diameter, but becomes smaller as it runs distally, until near the point 
of ending, where it again enlarges 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 
18u in diameter, and being bipolar are spindle-like in form. Their 
nuclei are spherical and from 10 yu to 12m 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 Paleemo- 
netes and Crangon, is not found in the nerve elements of the crayfish. 
3. The development of the otoryst was not studied in Cambarus. <Ac- 
cording to Reichenbach (’86) it is completely formed before the young 
animal leaves the egg. 
IV. Carcinus m=nas Leacu. (Green 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 meenas 
by Hensen (’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. 
