244 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
cell, extending from the neuropil of the brain nearly to the peripheral 
ending of the fibre. 
13. Each sensory ganglion cell with its central and peripheral fibres 
constitutes a single nervous element or neuron. ‘The neurons are 
trophic units, and direct connection between two neurons was not 
demonstrated. 
14. In those decapods which pass through free-swimming larval 
stages, the otocyst develops as an invagination of the dorsal ectoderm 
of the basal segment of the antennule, and becomes functional only at 
the fourth moult after hatching. 
15. Invagination begins at the second larval stage, but the matrix 
cells which are to form the sensory hairs of the sac, make their appear- 
ance in the first larva, being derived from the cells of the hypodermis. 
16. During the third stage the sensory hairs are formed below the 
floor of the shallow sac; at the next moult these become functional, 
the sac enlarges, and otoliths make their appearance. The otocyst 
is now functional, the hairs are innervated as in the adult, and more 
than 100 of them may be present. After the fourth stage the chief 
changes are the increase in the number of otocyst hairs, and the gradual 
constriction of the orifice of the sac. 
17. In Brachyura the Zoea larva is without a functional sac. In the 
Megalops the otocyst is open, and contains numerous sensory hairs and 
otoliths. During the next two stages the aperture closes and takes on 
the adult condition, without otoliths. 
18. Structurally, the otocyst of decapods may be compared roughly 
to the utriculus of such a vertebrate as Myxine; the sac of Paleemonetes 
to a single isolated ampulla, and its sensory cushion to a crista 
acustica. The closed otocyst of Brachyura has three sensory regions 
and is without otoliths. It therefore approaches in general structure 
the utriculus of the higher vertebrates. Each sensory element of the 
otocyst is comparable to a single sensory component in the vertebrate 
crista. In each there is a modified organ for the reception of stimuli, 
connected basally with the terminal fibre of a sensory neuron. 
19. There is no part of the decapod otocyst which is structurally com- 
parable to the middle ear, semi-circular canals, or cochlea of vertebrates. 
20. There is no direct evidence to prove that decapod Crustacea react 
to true sounds produced either in water or in air. The reactions 
formerly attributed to audition are probably due to tactile reflexes. 
21. The otocyst plays little or no part in calling forth these reac- 
tions, and does not function as a true auditory organ. 
