192 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



adults entirely, that is, without the aid of dissection or serial sec- 

 tions, and the same may be said of all the other investigators, whether 

 working with eggs, lai'vse, or adult stages. Since all of these are more 

 or less opaque it is not surprising that some mistakes were made in 

 observations. The wonder rather is that so many of these observa- 

 tions of the exterior are substantiated by sectioning the different 



stages. 



COMPARISON WITH OTHER PARASITIC SPECIES. 



On comparing carefull}^ and in detail the metamorphoses which the 

 different species exhibit, we may separate the parasitic copepods into 

 five groups according to the relative lengths of the free-swimming and 

 parasitic periods: 



I. Those whose larvae are free swimmers during their entire devel- 

 opment and become parasites only upon reaching the sexually mature 

 adult condition. 



The Ergasilidse are examples of this group, and in the genus Erga- 

 silus the males never become parasites at all but remain free during 

 life. 



II. Those in which the earlier larval stages and the mature adult 

 are free-swimming, while the intervening stages are parasitic and 

 sometimes degenerate. 



Here belong the MonstrillidoB and the males of certain genera 

 among the Ascidicolid£e. The males and often the females of some 

 of the Caligidse might also be well placed here, since the mature 

 adults are frequently captured swimming at the surface, and only 

 attach themselves temporarily to fish in order to obtain food. 



III. Those whose larvae are free swimming during early develop- 

 ment, passing through typical nauplius, metanauplius, and copepodid 

 stages, and then seeking a host upon which to become parasitic during 

 the remainder of life. 



The stage at which the change is made varies considerably, as does 

 also the life history subsequent to it. In some of the Ascidicolidae 

 the larvae remain free-swimming until the second copepodid stage, as 

 in Enterognathus. In the Pandarinse, Cecropinse and Chondracan- 

 thidae they become parasitic at the beginning of the first copepodid 

 stage. In the latter family they are transformed at the next molt 

 into the adult, the other copepodid stages being suppressed. 



IV. Those in which the early stages are often passed inside the egg, 

 while later free-swunming stages alternate with others which are 

 both parasitic and degenerate. 



The Lernaeidae are examples of this group; the larvae are free-swim- 

 ming until the first copepodid stage, then become parasitic and degen- 

 erate into a pupal form in which the power of movement is lost. 

 Later they regain this power, leave their host, and swim about freely 



