PKENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 215 



The otocj'st 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 larvae, 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 Ear. 



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 1 



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 cristae of higher vertebrates, and the sac itself resembles 

 the utriculus. But there is 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 Eetzius ('94) and Morrill ('98) that these 

 epithelial sense cells of the cristse in vertebrates are not true nervous 

 elements, 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. 



