326 PR0CEEDIN08 OF THE NATIONAL MUSEUM. vol. 39. 



stages must be present in the plankton at the right season and could 

 be obtained by towing, or they could be reared in suitable aquaria. 



It would be of particular interest to obtain the complete develop- 

 ment of both sexes, following especially the movements of the maxilli- 

 peds and the backward migration of the mouth-parts. The rest of 

 the details are doubtless quite similar to those found in free-swimming 

 forms. 



Behind the disappearing maxillipeds may be seen the rudiments of 

 the first two pairs of swimming legs (fig. 36, 1 and 2), which will 

 appear more completely developed in the first copepodid stage. 



Genus ERGASILUS Nordmann. 



Ergasilus, Nordmann, 1832, p. 7. 



Body cy clops-like, narrowed posteriorly; first thorax segment 

 fused with the head to form a carapace which is inflated dorsally, 

 especially in the female when the ovaries are well developed. Under 

 these conditions it usually overlaps and conceals the following free 

 segments, of which there are four, followed by the genital segment, 

 which is but slightly enlarged. Fourth free (fifth) segment very short 

 and not easily distinguished; abdomen also short, narrow, and 

 indistinctly jointed; anal lamina with very long setae. First anten- 

 nae six-jointed and well armed with setae; second pair stout, four- 

 jointed, very strong in the female, much smaller and weaker in 

 the male. Mouth nearly in the center of the carapace on the ventral 

 surface; mouth-parts already described (p. 311). Five pairs of swim- 

 ming legs; the four anterior pairs biramose, the fifth pair uniramose 

 and very rudimentary, sometimes apparently wanting. Egg-tubes 

 like those of Cyclops, eggs small and numerous. Adult females 

 parasites on the gills of (mostly) fresh-water fish, males always 

 remaining free. 



{Ergasilus, the name of a parasite in Plautus.) 



ARTIFICIAL KEY TO THE SPECIES. 



The following published species have been eliminated from the key for the reasons 

 stated. Biuncinatus Gadd, 1901, belongs to the genus TJiersitina; depressus Sars, 1862, 

 proves to be a synonym of sieboldii; esocis Sumpf, 1871, is another synonym of sieboldii. 

 Sumpf does not tell us the host of this species, but the name he has given it would 

 make it certain that it came from some species of Esox, which is also one of the hosts 

 of sieboldii. Fm-thermore in this portion of his paper Sumpf is discussing only Die 

 Mundwerkzeuge der sogenannten Poecilostomen, and he describes and figures 

 nothing but the mouth-parts of esocis, without any mention of its body form or other 

 appendages, or any further description of it as a separate species. Four years later 

 Glaus, from whom Sumpf obtained his material, published a description of sieboldii, 

 and his figure of the mouth-parts is printed from the same plate as this one of Sumpf 's. 

 Hence esocis must be regarded as a name wrongly given by Sumpf to sieboldii, under 

 the impression that it was a new species. Gasterostei, given by some authors as a 

 species of Ergasilus, belongs rather to the genus Thersitina (p. 349). Mugilis Vogt, 1877, 



