Hadley, Behavior of the American Lobster. 



293 



apparent that this difficulty appHes rather (at least in the reactions 

 of the larval lobsters) to those movements which determine the 

 progressive orientation to light, than to those which determine 

 body-orientation. Even in the latter somewhat less complicated 

 and more easily explained phenomena, however, we are still far 

 from recognizing the underlying causes. 



It is true that we can understand in a way why the "posterior 

 position" of the thoracic appendages determines a negative 

 response, while the "anterior position" determines a positive 

 response. We can, moreover, understand why a more intense 

 illumination of the eye on one side causes a greater activity of 

 the swimmerets on that side, and a consequent swing of the larva 

 away from that side. This phenomenon was well shown by experi- 



FiG. 22. Diagram showing the rostrum and one eye of a larval lobster; a, b, c, d represent direction 

 of light striking the eye from behind, below, in front and above; a.l.s. represents posterior lateral 

 surface; p.l.s. represents anterior lateral surface. For further explanation, see p. 297. 



ments which the writer performed upon larvae with blinded eyes 

 (Hadley 1908). These experiments demonstrated that, when 

 the right eye was blinded, the direction of forward swimming was 

 invariably to the right; in other words, the exopodites beat more 

 vigorously upon that side of the body whose eye was most stimu- 

 lated, and the larva was, in consequence, "pulled around" like 

 a boat. These reactions are explainable on the grounds of a 

 heterolateral stimulation and a consequent unequal action of the 

 muscles on the two sides of the body. But we do not understand 

 as clearly how or why the action of the light striking with equal 

 intensity the corresponding areas of the posterior surface of the 

 eyes (Fig. 22), for instance, brings about these "anterior" or 

 "posterior" positions of the thoracic appendages, and the con- 



