354 
bulletin of the bureau of fisheries. 
It is evident from the preceding chapter, as Hadiey has already pointed out, that 
the life of the lobster may be divided on the basis of behavior into three periods: (i) 
The three larval stages, when the animals frequently swim with head depressed, upward 
or downward and forward or backward, according to circumstances, by the use of their 
thoracic exopodites; (2) the fourth stage, when the animal is a free swimmer at the 
surface, the abdominal swimmerets being now functional, as in the adult; and (3) the 
later stages, when the swimming organs are the same, but the animal remains constantly 
on the bottom after its final descent in the fourth or fifth period. 
REACTION TO LIGHT. 
The response of the pelagic larvse of the higher Crustacea to light, as well as 
the effect of light upon the growth of these animals, are questions not only of great 
scientific interest, but in the case of the lobster of practical importance in view of 
the necessity of understanding their behavior in a state of nature and of placing 
them as far as possible under natural conditions in the hatchery. It has been 
shown in general that swimming larvae of crustaceans, in common with many other 
organisms, exhibit two types of response to the light stimulus, known as phototaxis 
or reaction to the directive influence of the rays of light and photopathy or response to 
changes in the intensity of light. The phototactic response is composed of two ele- 
ments or components — the turning and progressive movements or, as Hadley calls 
them, the body and progressive orientation; the animal turns so that the long axis of 
its body coincides with the path of light, and it always heads away from the source; 
this reaction is primary, constant, and typically reflex. On the other hand, the “pro- 
gressive” response which follows this stereotyped form of orientation may be positive 
or negative — that is, the animal may move upward or downward, backward or forward — 
that is, toward or away from the source of light. The photopathic response is also 
variable, the animal moving toward or from a more brilliantly illuminated region, 
according to conditions. 
Thus, according to Hadley, apart from the orientation of the body there is no 
constant type of reaction for the larval lobster. The variable responses vary in accord 
with changes in the environment of the individual and changes in the individual itself 
or its physiological state, and are especially marked at the beginning and close of the 
stage periods. While the phototactic response is eminently variable, the photopathic 
reaction is usually positive. 
In the fourth stage the conditions are somewhat reversed, since in the laboratory 
lobsters at this period usually give a negative phototactic reaction, while their photo- 
pathic response is at first positive and later negative. Tight-avoiding reactions of 
whatever kind are strongly manifested in the fifth stage and may begin at the close of 
the fourth. So strong indeed was the tendency to shun the light that the little lob- 
sters, as Hadley demonstrated, would even allow themselves to be stranded, with pos- 
sible fatal results, rather than to approach the light, and thereby gain deeper water. 
It was further shown that at this time also the thigmotactic reaction, or response to 
