Hadley, Behavior of the Ajnerican Lobster. 263 



progress of the larvae may often be directly forward or directly 

 backward with but slight deviation from the horizontal plane; 

 while at other times, when the stroke of the exopodites is directly 

 outward and downward (exclusive of either the "forward" or 

 "backward" factor), the larvae may mount to the surface in nearly 

 vertical lines. 



It thus becomes evident that the progression of the larvae, back- 

 ward or forward, upward or downward, is largely determined by 

 the position (state of extension or contraction) of the thoracic 

 appendages. In other words, if for the greater part of the time 

 these appendages are in the "anterior" position the phototactic 

 reaction of the larva is positive; but on the contrary, if the thoracic 

 appendages are more frequently in the "posterior" position, then 

 the consequent reaction of the larvae is negative. Naturally the 

 next important question which arises is: What conditions deter- 

 mine the "anterior" or the "posterior" position of the thoracic 

 appendages .^ It cannot be questioned that these changes are 

 directly due to certain variations in the intensity of the illumination 

 and are modified by the "physiological state" of the larvae them- 

 selves; and that, furthermore, the state of extension or contraction 

 of the thoracic appendages, and the stroke of the exopodites, are 

 regulated to a great degree through the mediation of the eyes and 

 the nervous system of the larvae. But further consideration of 

 this subject must be postponed until later. In the meantime we 

 may turn our attention to the mechanics of body-orientation. 



J. The mechanics of hody-orientation — Under the present 

 heading we shall consider the nature of those peculiar movements 

 which the lobstef larvae undergo when they are under diverse and 

 changing conditions of stimulation, in order to explain the cause 

 of these actions and to show their relation to certain definite laws 

 which may be said to regulate to a great degree the body-orienta- 

 tion of the larvae. As we have observed, it is the influence of 

 light which is most active in determining the behavior of the larvae; 

 furthermore, it is in the absence of such influences as diverse and 

 changing conditions of illumination afford that the most realistic 

 picture of the normal behavior of the larvae is obtained. It will 

 then prove the most practical method of approaching this problem, 

 first, to obtain conditions of light which allow natural behavior 

 (normal swimming); and then, by gradually modifying these con- 

 ditions, to observe the effects upon the behavior of the larvae. 



