PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 215 
The otocyst of Carcinus thus resembles very closely in its development 
that of the lobster. In both there is no trace of the organ in the newly 
hatched larvee, and for three successive moults it is not functional. In 
the fourth larval stage, with a sudden metamorphosis of the animal’s 
general form, the otocyst is also rapidly changed from a mere depression 
to an active, well-developed organ. The significance of these sudden 
correlated transitions will be seen when the otocyst is considered 
physiologically. 
C. THEORETICAL CONSIDERATIONS. 
1. Comparison of the Otocyst with the Vertebrate Har. 
The otocyst has been compared by many investigators to the auditory 
organ of vertebrates. Leaving their functions entirely out of account, 
how far do the two correspond in structure ? 
The otocyst of Macrura consists of an open sac, a sensory prominence, 
bristles, and otoliths resting upon them ; essentially the-same conditions 
as are found in the ear of Myxine, though the latter has five sensory 
regions instead of one. The otocyst of macruran decapods might thus 
be well compared to an isolated ampulla in the ear sac of Myxine, and 
the sensory cushion to a single crista acustica. 
In the Brachyura the organ is still more highly differentiated. The 
sac is closed, there are three sensory regions, and the hairs found on 
them project free into the lumen of the otocyst; otoliths are entirely 
wanting. The structure of the sensory apparatus is in this case similar 
to that of the cristz of higher vertebrates, and the sac itself resembles 
the utriculus. But there zs no portion of the decapod otocyst so differen- 
tiated as to bear more than a fancied resemblance to the semicircular 
canals, the middle ear, or the cochlea of higher vertebrates. 
Each crista acustica in vertebrates, however, is made up of separate 
elements, which may be compared to the sensory elements of the otocyst. 
Every auditory hair of the crista is developed from the exposed end of a 
specialized epithelial sense cell, which itself forms the basal part of the 
hair, and is supported in position by the other cells of the epithelium. 
It has been shown by both Retzius (’94) and Morrill (’98) that these 
epithelial sense cells of the criste in vertebrates are not true nervous 
elernents, as the auditory fibres are not continuous with them. Both 
the cell and its auditory hair taken together are to be compared to the 
bristles of the otocyst, in that they constitute a non-nervous end-organ. 
