NATURAL HISTORY OF AMERICAN LOBSTER. 



239 



From the foregoing account it wUl 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 {131), 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 anteimse, 

 may help a crustacean to maintain its erect attitude, the sacs are indispensable for this 

 purpose, at least after the larval stages. 



sac—' 



Fig. 



sr 



—Sectional view of antennal segment to show statocyst, with needle 

 inserted in pore at surface and pointed to sensory ridge, s t. 



