232 ^Journal of Comparative Neurology and Psychology. 

 J. Third Larval Stage. 



By the ninth day it is only in exceptional cases that the larvae 

 have not entered the third stage; and it frequently happens that 

 they are nearly ready to enter the fourth. The swimming of the 

 third-stage larvae is much like that of the earlier stages except that 

 in the third stage there is greater difficulty in using the swimmerets 

 of the thoracic appendages, especially during the last part of the 

 stage. One reason for this is the fact that, as the larvae grow 

 older and larger, they more often play the host to multitudes of 

 diatoms, algae and protozoa which gather in such quantities as 

 seriously to interfere with the processes of swimming and eating. In 

 the preparation for the moult from the third to the fourth stage, 

 moreover, occur the most important changes that the young lob- 

 ster undergoes in the course of its life. These changes appertain 

 not alone to modifications in the external form of the body and to 

 the form and functions of many of the body appendages, but also 

 to points of internal structure. Among the changes during this 

 period of metamorphosis we may enumerate the following as 

 important in connection with our study of behavior: (i) The loss, 

 in the moult from the third stage, of all functional swimming 

 attachments of the thoracic appendages; (2) the great develop- 

 ment of both the first and second pairs of antennae and of the 

 chelipeds; (3) the accession of functioning swimmerets on the 

 under side of the second to sixth abdominal segments; (4) a great 

 change in the form of the body, and a consequent modification 

 of the manner of swimming. 



In view of these important changes, which are taking place in 

 the anatomy of the lobsters as they pass from the third into the 

 fourth stage, it does not appear unjustifiable to believe that these 

 processes have an influence on the behavior of the larvae even before 

 they emerge in approximately the adult structural type, endowed 

 with a new body form, new functional apparatus and new reac- 

 tions. We shall now undertake a study of the behavior of the 

 third-stage larvae as they approach and finally pass this most crit- 

 ical period of their life history. 



Experiment 21. Case i — July 22, 9 130 a.m. Thirty ninth-day, 

 third-stage larvae were removed from the hatching bag, put in the 

 glass jar and placed in the dark box. Under stimulation by the 

 red rays, although there was no definite positive reaction, most of 



