424 On the Parasitic Copepod Tliersitina gasterostei. 



Stage 6 (PI. XIII. fig. /,<?): 



Free-swimming adult. 



In both sexes the adult form is assumed for all ap- 

 pendages, and the abdomen in the male becomes 

 five-jointed. 



Systematic Position. 



Wilson * has already discussed the relationship of Tliersi- 

 tina and Ergasilus, and has come to the conclusion that the 

 two genera are distinct ; and with this conclusion I am in 

 agreement. The differences between the genera are, how- 

 ever, very small, the most important being the inclusion of 

 the second thoracic segment in the globular cephalothorax 

 of the adult female and the structure of the antennas. The 

 mouth-parts differ somewhat in structure, but are the same in 

 number as in Ergasilus. 



The genus contains but one species, T. gasterostei (Pagen- 

 stecher), since T. hiuncinatus (Gadd) is certainly indistin- 

 guishable from it. 



EXPLANATION OF THE PLATES. 

 Plate X. 



Tliersitina gasterostei, mature female. Leg 1 on right side aud 

 legs 2 and 3 on left omitted. 



Plate XL 



Fig. a. Ventral view of male (adult). 

 Fig. b. Ventral view of female (free-swimming adult). 

 Fig. c. Mouth-parts of adult egg-bearing female. The second maxilla of 

 the right side removed and the first maxilla somewhat displaced. 

 Fig. d. First maxilla isolated. 

 Fig. e. First and second maxillae isolated. 



Tlate XII. 



Fig . 1. Abdomen of larva in stage 4 and 5. a, a', stage 4, male and 



female ; b, b', stage 5, male and female. 

 Fig. 2. First Nauplius immediately after hatching. 

 Fig. 3. Second Nauplius six days old. 

 Fig. 4. Third Nauplius eleven days old. 

 Fig. 5. Fourth Nauplius. 



Plate XIII. 



Figs. a-e. Five Cyclopid stages. 

 Fig. f. Adult male. 



* Wilson, Proc. U.S. Nat. Mus. xxxix. 1911, p. 349. 



