266 PROCEEDINGS OP THE NATIONAL MUSEUM. vol.39. 



While the adult females become more or less fixed upon their host, 

 the males remain free-swimmers during life, and at least in the genus 

 Ergasilus do not appear to practice parasitism at all. In consequence 

 of this difference there is alwa^^s a great scarcity of males, especially 

 among the Ergasilinge. 



Females can be found in abundance upon nearly all our common 

 food fishes, but the males disappear at the close of the mating season 

 and can then be found only in the tow. 



We can thus understand how in the great majority of species the 

 female alone is known. In a very few instances the two sexes are 

 found together, as noted by Mr. T. Scott, an excellent investigator 

 and one who has worked upon free-swimming as well as parasitic 

 forms. In a short paper published in the Annals of Scottish Natural 

 History (vol. 9, p. 153) he calls attention to the fact that certain 

 species of Bomoloclius are habitually found in the nostrils of such fish 

 as the cod {Gadus callarius), the lumpsucker (Cyclopterus lumpus), 

 and the plaice (Pleuronectes platessa). Both sexes and the young 

 live here in the mucus lining of the nostrils, and when removed to an 

 aquarium prove to be lively swimmers. 



The mating of the sexes in this family takes place while the female 

 is still very young and in all probability before she seeks out a host. 

 And only at that time would there be any chance of finding the two 

 together. 



The evidence for these conclusions is contained in the following 

 facts: The developmental stages of all the Ergasilidas are free- 

 swimming; none of them are found attached to a host along with the 

 adults, as is the case with the chalimus stage of the Caligidse. Among 

 the many hundreds of specimens taken by the author from the gills 

 of different fishes in both salt and fresh water no developmental 

 stage has ever been found. Undersized females have been repeatedly 

 obtained, less than half the length of the fully developed adult, and 

 giving unmistakable proof that they had not as yet developed their 

 first pair of egg strings. But they were still sexually mature, the 

 eggs within the ovaries were well advanced, and the spermatophores 

 were already attached to the genital segment. 



Again, in all this large number of specimens, as well as among those 

 contained in the National Museum, there have never been found a 

 male and female in union. And there is but a single record of such 

 a find within the author's knowledge, the one given by Bassett- 

 Smith (1898, p. 358) of Bomoloclius megaceros. If the coming together 

 of the sexes took place upon the gills of the host many more would 

 undoubtedly have been obtained, as among the Caligidse. That they 

 have not been thus found indicates that mating takes place before 

 the female seeks a host. Fortunately the few males known are well 

 distributed among the genera, and we thus have a good idea of that 

 sex throughout the family. 



