Parasitic Copepod Thersitina gasterostei. -123 



Stage 1. First Cyclopid (PI. XIII. fig. «) : 



Antenna 1 indistinctly four-jointed. 



Antenna 2 five-jointed as in the adult, luit without 



accessory claws. 

 Mandibles and maxillae as in the adult. 

 Leg 1 with indications of two joints in each ramus. 

 Leg 2 with one-jointed rami. 

 Leg 3 simple papilla?. 

 Leg 4 indicated beneath the skin. 

 Body : four thoracic segments distinct ; posterior region 



unsegmentcd. 



Stage 2 (PI. XIII. fig. b) : 



Antenna 1 distinctly four-jointed. 

 Antenna 2 and mouth-parts as before. 

 Legs 1 to 3 with two-jointed rami. 

 Leg 4 simple papilla?. 



Body : five thoracic segments distinct ; abdomen un- 

 segmented. 



Stage 3 (PI. XIII. fig. c) : 



Antenna? and month-parts as before. 

 Legs 1 to 3 with two- jointed rami. 

 Leg 4 with rami one-jointed. 

 Leg 5 indicated by a seta, but no papilla. 

 Abdomen of two segments. 



Stage 4 (PI. XIII. fig. d) : 



First differentiation of sex. 



Appendages as before, with the exception that the fifth 

 pair of legs is present and in the male the maxilli- 

 pede is present as an unjoin ted papilla. 



Abdomen three- jointed (PI. XII. fig. 1, a, a'). 



Stage 5 (PI. XIII. fig. e): 



In the male maxillipede with indication of joints. 



In both sexes leg 4 two-jointed. 



Abdomen four-jointed (PI. XII. fig. 1, b, //). 



In the female the first abdominal segment has increased 



in size. 



