PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. LV: 
sidering the otoliths simply particles of sand; for sometimes the sacs 
are closed, and again the openings are often too smal] to admit the 
passage of the otoliths from the exterior. They must be, then, deposits 
of calcium carbonate secreted by the animals themselves. 
Hensen’s (’63) account of the otocyst is far more complete than any 
other, and a fairly extensive review of his paper is necessary for the 
sake of later comparisons. He worked mostly with freshly collected 
animals, although some twenty-four species were studied from alcoholic 
material. His principal methods were dissection and maceration, some 
few crude sections, however, being made. The paper is divided into an 
anatomical and a physiological part. The latter portion will be re- 
viewed, along with other papers of a similar nature, in Part II of this 
paper. 
The elementary parts of the typical auditory organ are described by 
Hensen (63, p. 326) thus: ‘Der Gehdrapparat der héheren Krebse 
besteht nun, kurz gesagt, darin, dass, von der Endganglie eines Nerven 
ein feiner Faden in ein Chitinhaar hineintritt, und an einen eigen- 
thiimlich gebildeten Theil der Haarwand sich festsetzt. Diese Haar- 
wand ist so locker mit der Schalenhaut verbunden, dass sie bei 
entsprechenden Ténen recht bedeutende Schwingungen vollfiihren kann 
und vollfiihrt. Das Haar selbst geht zuweilen noch in oder zwischen 
Steine hinein.” 
Crustacea he divides into four classes according to the condition of 
otocyst and otoliths :— 
1. Sacs closed, with one otolith : example, Mysis. 
2. Sacs closed, without an otolith: all Brachiura. 
3. Sacs open, many otoliths : Astacus, Paleemon. 
4, No sac nor otoliths, but free auditory hairs. 
Otoliths. In confirmation of Farre it was found that the otoliths of 
decapods having open ear sacs were mainly composed of grains of sand. 
This was proved by chemical tests, and by keeping newly moulted 
animals (Palemon) in filtered water to which uric acid crystals had 
been added. Examination of the otocysts some time after moulting 
showed the presence of these crystals in the sac. In larger forms, such 
as the lobster and crayfish, the sand particles are spread over the whole 
basal surface of the ear sac. In shrimps and prawns they are more 
closely aggregated. The single otolith found in Mysis flexuosus is 
described at length, but as this account has been corrected by Bethe 
(’95), it will be referred to later in connection with Bethe’s work. 
The Otocyst (Hérblase of Hensen) is described in general as a round- 
