NATURAL HISTORY OF AMERICAN LOBSTER. 
239 
From the foregoing account it will be seen that in the water-filled sacs just described, 
with their rich supply of sensory hairs, many of which, having little weights in the form 
of sand grains glued to their tips, and all being subject to the impact of free particles 
with the least displacement of the body, we have what would seem to be an admirable 
apparatus for enabling the animal to carry itself erect in walking or swimming. Any 
swaying of the whole body would sway the little hairs, or rattle the sand over them, 
and the stimulus thus given, would act as a sign to which the nervous system of the 
animal could respond in an adaptive and useful manner. 
The study of development throws some light on the probable use of these peculiar 
sense organs. As shown by my earlier studies but first carefully worked out with 
histological definiteness by Prentiss, the sacs are developed in the free-swimming stages. 
They are barely visible as shallow depressions in the second and third larvae, but in the 
fourth stage sensory hairs and sand grains are present, and closure of the sacs, which 
has now begun, is gradually effected with each successive molt. As Prentiss has shown, 
this “sudden leap” in the appear- 
ance of the sacs at the fourth stage 
is probably related to the abrupt 
change in form and method of 
swimming exhibited at the fourth 
molt. 
Every one who has watched 
the swimming movements of the 
young lobsters up to the fourth 
stage (fig. 34 and 42) has noticed 
how unsteady they become when- 
ever the water is in the least de- 
gree disturbed. In ordinary swim- 
ming, when their equilibrium is not 
upset, the thorax is horizontal and the abdomen bent; in rising the head is 
inclined downward, but at best they are very unstable, and frequently pitch and 
reel to and fro, swimming now on their backs, now with their heads directed up 
or down. (See fig. 40.) It should be added, however, that under certain conditions, 
as in dull light, the young larva, as Hadley observes (iji), swims with grace and pre- 
cision, and there is no doubt that the eyes act before the statocysts as organs of 
orientation. 
At the fourth stage (pi. xxxi) the little animals uniformly bear themselves erect like an 
adult and move about with great speed and definiteness. Prentiss has pointed out that 
when the young at this stage are unable to get sand for the statocysts, their movements 
again become uncertain, like those of an adult animal from which the sac has been 
removed. It is thus evident that while other organs, such as the eyes and antennae, 
may help a crustacean to maintain its erect attitude, the sacs are indispensable for this 
purpose, at least after the larval stages. 
Fig. 3. --Sectional view of antennal segment to show statocyst, with needle 
