of the Fishery Board for Scotland,. 



183 



were practically the same species, and that the differences observed in 

 their form and structure were simply the result of the difference 

 in the mode of life they had happened to adopt. One might, in support 

 of such an explanation, point to the close resemblance to each other of the 

 young of the so-called different species — a resemblance so close that at 

 this early stage it is difficult even for an expert to decide in some cases 

 which belongs to one species and which to the other. 



It must also be admitted that in the adult forms of certain species 

 modifications are observed which have a provoking tendency to bridge 

 over the space that intervenes between one species and another, and 

 which are exceedingly troublesome to the systematic. 



Notwithstanding all this, however, the explanation I have referred to 

 can scarcely be considered tenable, except to a very small extent. There 

 are too many difficulties in the way of accepting such an explanation ; 

 but it shows that even in a limited subject like that of the parasites of 

 fishes there is still work for the theorist as well as for the student who 

 simply deals with the practical aspect of the study. 



DESCRIPTION OF THE PLATES. 



PLATE V. 



Thersites gasterostei, Pagen. 



Fig. 1. Female, dorsal view . . . . . x 40. 



Fig. 2. Antennule ....... x 380. 



Fig. 3. Antenna . . . . ... . x 253. 



Fig. 4. Posterior foot-jaw . . . . . x 833. 



Fig. 5. Foot of first pair ...... x 253. 



Fig. 6. ,, fourth ,, . . . . . x 253. 



Fig. 7. Abdomen, dorsal view . . . . . x 127. 



Oaligus curt us, Miill. 



Fig. 8. Female, dorsal view . . . . . x 48. 



Fig. 9. Antenna . . . . . . . x 53. 



Fig. 10. Sternal fork . . . . . . x 53. 



Fig. 11. Second foot-jaw . . . . . . x 40. 



Fig. 12. Foot of fourth pair . . . . . x 18. 



Caligus rapax, M. -Edw. 



Fig. 13. Female, dorsal view. . . . . . x 7. 



Fig. 14. Male ,, ...... x 7. 



Fig. 15. Mandible . . . . . . x 166. 



Fig. 16. One of the palpi — male . . . . . x 96. 



Fig. 17. Sternal fork . . . . . ' . x 80. 



Fig. 18. Posterior foot- jaw . . . . . x 40. 



Fig. 19. Foot of fourth pair . . . . . x 40. 



Caligus diaphanus, Nordm. 



Fig. 20. Female, dorsal view . . . . . x 1 1 *4. 



Fig. 21. Male ,, ...... x 19. 



Fig. 22. Mandible ....... x 253. 



Fig. 23. Sternal fork ....... x 160. 



Fig. 24. Posterior foot- jaw . . . . . x 80. 



Fig. 25. Foot of fourth pair . . , , , x 80. 



