PRENTISS : THE OTOCYST OF DECAPOD CKUSTACEA. 183 



old bristle for the purpose of forming the new hair (Fig. 9, cl. ma.). 

 This aggregation of cells is similar to the -paxAlla described by Braun, 

 but they are by no means as regular in outline and arrangement as 

 those figured by him. lu Palaemonetes this condition of the matrix 

 cells exists for several weeks before ecdysis takes place, the new hairs 

 being formed during this period. In adult lobsters and crayfish the 

 time is probably much longer, whereas in larvae it lasts but a few days. 

 The chitin of the new hair shaft is secreted pari j^assu with that of the 

 test, so that the two are continuous, but the hew hair is beneath the 

 shell, in the region where the matrix cells have formed the papilla. It 

 is secreted as a double tube, the distal end of the inner part of which 

 projects, as the tip of the new hair, into the base of the old one. 

 Figure 10 (Plate 3) shows the condition of affairs just before ecdysis in 

 the endopod of the third abdominal appendage ; eta. being the old test, 

 da. the new one formed beneath it. Three newly formed hairs are seen 

 as double tubes located deep in the appendage. The walls of the two 

 tubes are continuous with each other at their lower or proximal ends, 

 and the tip of the inner tube projects distally into the shaft of the old 

 hair. This inner tube, the tip of the new hair, must be secreted by the 

 delicate processes from the matrix cells which still extend np into the 

 old hair during the period of hair formation. The outer tube, though 

 continuous at its lower end with the inner, is secreted by two parallel 

 rows of matrix cells, very similar to the chitinogenous cells of the hypo- 

 derrais, and probably derived from them. Hensen ('63, p. 375) has well 

 described this condition of the new hairs as resembling that of the finger 

 of a glove turned partially inside out, the tips alone projecting. The tip 

 of the new hair is embedded in a viscous, homogeneous substance, which 

 is formed between the old and the new tests, either by glandular secre- 

 tion of other cells or by the chitinogenous cells themselves. This 

 substance probably corresponds to the homogeneous non-cellular mem- 

 brane found by Herrick between the shells of the lobster ('95, p. 87). 

 ^\ hen the old test is shed, it adheres to the fine plumes of the new hair 

 tip, and aided by the internal blood pressure (very considerable at the 

 moulting period), draws the recently formed hair out into its functional 

 position, just as one would draw out the invagiuated finger of a glove by 

 pulling on its tip. The chitin of the shaft is very soft and pliable at 

 this time, allowing the hairs to be turned right side out with ease ; 

 uideed, this may be done artificially. But if by some accident at the 

 time of ecdysis any of the hairs are not at once fully drawn out, the 

 chitin hardens and they are fixed in their abnormal position. 



