188 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



reaction. As was also the case in the previous stages, the photo- 

 tactic response appeared to be the dominating factor in determining 

 the behavior of the larvse in the third stage, although the reactions 

 of the third-stage larvae were, as a rule, much less definite than the 

 responses of larvie in the earlier stages. 



Fourth-Stage Lobsters. 



In the molt from the third to the fourth stage occur the most 

 important changes that the young lobster undergoes in the whole 

 course of his life. These changes appertain not alone to modifica- 

 tion in the body form, morphology, and function of many of the 

 appendages, but also to points of internal structure as well. Among 

 the more general features of the metamorphosis we may enumerate: 

 (1) The loss, in the molt from the third stage, of all functional swim- 

 ming branches on the thoracic appendages. (2) The great develop- 

 ment of the chelipeds and of the first and second pairs of antennae. 

 (3) The accession of functioning swimmerets on the underside of the 

 second, third, fourth, and fifth abdominal segments. (4) A great 

 change in the body form, — and consequently, in the manner of 

 swimming. 



In view of the importance of these changes which take place when 

 the lobster enters the fourth stage, it is reasonable to expect that 

 they are to have some influence on the behavior of the fourth-stage 

 lobster. And we shall find that, just as a certain change in the 

 nature of the response was shown during the progress of each of the 

 earlier stages, so in the fourth stage, but for diff"erent reasons, there 

 is a gradual modification of the manner of reaction as the lobster 

 passes on through the fourth stage and enters the fifth. 



The phototactic reaction after the molt from the third stage was 

 found to be negative. We have observed that, in the earlier stages, 

 this negative reaction changed to positive as the molting period to 

 the next stage approached. This was not found to be the case in the 

 fourth-stage larvae: for, as the stage-period advanced, the photo- 



