462 ROBERT GDRNEV 



Habits of the Larvae. 



The zoeea of Corystes is probably the same as that 

 described by Weldon (1889), and figured as " a Portunid 

 zoa3a/^ though it differs from his figure in several respects. 

 Weldon came to the conclusion that the long spines have the 

 function of directing the movements; and enabling the animal 

 to swim rapidly in a straight line. There can be no doubt 

 that his conclusion is correct, for the larva always moves in 

 the direction of the long axis of the spines. This is well 

 shown when a zoa3a is caught in an eddy in the plunger 

 jar. It then makes a violent effort to escape, darting 

 upwards, sideways, or straight to the bottom, according to 

 the position of the spines at the moment. Normally the 

 larva rises and falls in the water, swimming upwards ver- 

 tically for a short distance, and then resting. In the 

 plunger jar larvee have a tendency to collect, especially the 

 younger stages, at the surface against the side of the jar. 

 Here they often push their dorsal spines through the surface 

 film, and are hereby suspended. Sometimes they rest against 

 the rod of the plunger, suspended in this manner, and to this 

 habit I have owed the death of several specimens which were 

 caught and drawn up upon the plunger rod. If a light is 

 brought to the jar at night the z ore as become extremely 

 active, swimming rapidly towards the light, the dorsal spine 

 directed forwards. The body may, however, be in any 

 position with regard to the axis of the spines, the animal 

 swimming upon its back, side, or ventral surface. 



It is of interest to note, in considering the function of the 

 spines, that the period of their presence corresponds exactly 

 with the period of the absence of an "auditory" organ. 

 'I'lio latter, as Prentiss (1901) has shown, is not developed 

 functionally till the Megalopa stage, when only traces of the 

 spines remain. There is a rapid reduction in the length of 

 the spines as compared to the total length of the body in the 

 third and fuurth larval stages, and it is in the last zoa!a 



