440 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ficial connection with the recess but opens onlv a short distance into 
the exoskeleton as a simple cleft from the bottom of which a faint 
useless crack leads dorsally. The small cavity shown in this figure 
is merely the extreme posterior end of the wider space seen in figures 
8 (pi. 43) and 27 (pi. 47) which in life is full of connective tissue and 
lies between the posterior end of the trumpet and the posterior face 
of the annulus. Compare figures 2, 5, 6 (pi. 43), 11, 14 (pi. 44), 27 
(pi. 47). 
To summarize, w T e may say that the exoskeleton covering the annu¬ 
lus furnishes a thick-walled case about a long bent tube which opens 
to the exterior at the anterior end and enlarges as a two-horned pouch 
at its posterior end while along its entire length it communicates with 
the surface by curved slits leading to the zigzag Suture. Essentially 
the annulus is a bent pocket lined by exoskeleton. 
When the crayfish casts off its exoskeleton the above described shell 
of the annulus is thrown off as part of the entire exoskeleton. When 
the shell is examined from the outside, the annulus looks complete 
and as above described. From the inner or dorsal view the annulus 
of the shell is also as above described, but as it is still part of the entire 
shell, it does not present exactly the same face dorsally as that shown 
in figure 2 (pi. 43). Lying deep down between the endophragmal 
chambers that lodged the muscles of the bases of the fourth and fifth 
legs the annulus inclines downward anteriorly so that its curved ridge, 
or trumpet, is much foreshortened and the pits leading into the tuber¬ 
osities point posteriorly and are not visible. Anteriorly the annulus is 
still hinged to the saddle-shaped sternum of the fourth somite while 
posteriorly it is continuous with the thin exoskeleton that runs from it, 
loosely, to the protuberant ridge across the sternum of the fifth somite. 
In one specimen observed just after casting off the old shell the new 7 
shell of the annulus w T as translucent except for small chalky white 
opaque areas on the promontory and v 7 as colorless except that the 
tvrn tuberosities were red. 
Turning now to the living part of the annulus w 7 e find it to be a 
conical mass of connective tissue covered by epidermis (pi. 44, fig. 
11, text-figures B and C). The connective tissue is the core of the 
annulus and is covered over by the epidermis which follows all the 
protuberances and recesses of the exoskeleton, so that the above described 
exoskeleton of the annulus is but the hardened outer surface of the 
annulus, folded in wdiere the epidermis is folded in, and related to the 
connective tissue and epidermis as the crust of a loaf is to the crumb. 
