PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 171 



sidering the otoliths simply particles of sand ; for sometimes the sacs 

 are closed, and again the openings are often too small 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 th.an 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 Gehorapparat der hoheren 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 

 eutsprechenden Tonen recht bedeatende Schwingungen vollfuhren kann 

 und vollfuhrt. Das Haar selbst geht zuweilen uoch 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, Palaemon. 



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 nevvly moulted 

 animals (Palaemon) 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 (Horblase of Hensen) is described in general as a round- 



