422 MEMOIRS OF THE CARNEGIE MUSEUM 
ney, and will remain in position, thus favoring the construction of a “ well-built”’ 
chimney. On the other hand, when the mud is very soft, chiefly so when the craw- 
fish is not digging new holes, but only cleaning out the old ones, the pellets are not 
firm, and the more liquid mud will flow down the outside of the chimney and ren- 
der it lower and broader and, consequently, less ‘ well-built.” This latter fact also 
explains why young specimens often construct the neatest and most elaborate chim- 
neys (Abbott). Young specimens, when they begin to work, bring out undisturbed, 
firm, and sticky clay, and the pellets are moré likely to remain where they are 
placed on the rim of the chimney, which thus becomes very regular. Old speci- 
mens, on the contrary, live in holes which are practically finished, and when they 
work it is rather a process of ‘“ housecleaning” than of ‘ housebuilding.” The mud 
removed is more liquid and less sticky, and thus the chimneys are shapeless and 
irregular. 
Very often the opening of the chimney is found closed. Abbott believes that 
the closing is merely the result of the accidental falling in.of the rim. This may 
‘indeed happen, but in other cases it is plain that the crawfish closes the aperture 
intentionally, and Girard (1852) regarded this as the completion of the work of 
chimney-building. Shufeldt and Harris likewise believe that the crawfish itself 
seals up the burrow. This is my own opinion, and with Girard I think that the 
sealing up is the final act characterizing the completion of the burrow. Sealed 
burrows are very often found (see Pl. XL, Fig. 9; Pl. XLI, Figs. 2, 3, 4), chiefly in 
summer and fall, and it is in many cases evident, by the material used (see PI. 
XLI, Fig. 6) that the shutting up was done by the crawfish by depositing pellets 
in the orifice. Often the “stopper” is not at the orifice itself, but a certain distance 
(5 to 6 inches) below. In fall the stopper is made rather substantially and fills the 
upper end of the hole for a distance of 6 to 10 inches (see Pl. XL, Fig. 9b at D), and 
such a filling cannot be accounted for by accidental falling in. 
In my opinion the construction of the hole is the chief aim for which the craw- 
fish works. For the removal of the clay and dirt an opening on the surface is 
needed; but when the burrow is completed this opening is shut up again, and the 
crawfish is content to remain inside, possibly for weeks or even months. This 
affords protection for the crawfish and its young from enemies (snakes). Females 
with eggs or young are almost always found in closed holes. It also affords neces- 
sary quiet and seclusion during the moulting process (soft shells are generally found 
in closed holes). It furthermore protects the hole from the disturbing influences 
of rain and frost. Of course it would not be advantageous to have the hole perma- 
nently sealed, since the crawfish wants to get out now and then (for mating, for 
