PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 183 
old bristle for the purpose of forming the new hair (Fig. 9, cl. ma.). 
This aggregation of cells is similar to the papilla described by Braun, 
but they are by no means as regular in outline and arrangement as 
those figured by him. In Palemonetes 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 larve it lasts but a few days. 
The chitin of the new hair shaft is secreted part passu 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, 
cta’. 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 up 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- 
dermis, 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). 
When 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 invaginated 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; 
indeed, 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. 
