354 BULLETIN OF THE BUREAU OF FISHERIES. 



It is evident from the preceding chapter, as Hadley 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 larvae 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 crusta ceans, i n common with many otj jpr^ 

 organis ms, exhibit two types of response to the light stimulus, known as phototaxis 

 or reaction to the directive influence of the ravs of light and photopathy or response to 

 changes in the intensity of Ught. The phototactic response is composed of two ele - 

 ments or com ponents — the turning and progressive movements or, as Hadley calls 

 t hem, the body and progressive orientation; the animal turns so that the lonp; axis nf 

 i ts body coincides with the path of light, and it always heads away from the source: 

 this reaction is prima ry, constant, and typically reflex. On the other hand, the "pro- 

 gre ssive" response which follows this stereotyped form of oripntatinn may hp po^itivp 

 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, accordin g to Ha.rilpy, apart from thp nrientatinn of thp hodv there i.-^ nn 

 rrpffiant tvpe of re action for the larval lohstpr 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 fou rth s tage the conditions a re some what reversed, since in the laboratory 

 lobsters at this period usually give a negative phototactic reaction. whileThpir phntg- 

 pathic response is at first positive and later nes;atye. Li^ht-avoiding^ reactions of 

 whatever kind are strongly manifested in the fifth stage and may begin at the close o f 

 the fourth. So strong indeed was the tendency to shun the light that the little lob* 

 s t^rs, as Hadley de mon.stra.terl. woiiH pvpn gllnw HiPtncpl vps to hp stranHpH witVi pq g- 

 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 



