652 pkocEEbiNGS of the national museum. 



Thoy fastened upon the ventral surface and upon the sides of the 

 body near the operculum; the dace and roach resented the attack and 

 thrashed around vigoroush', Init to no avail; the bream accepted the 

 situation without protest. After remaining in the aquarium over 

 night the dace and bream were found dead and floating around with 

 exactly the same appearance as the dead fish in the pond. This, of 

 course, suggested that if the number of parasites was sufficient in 

 the pond, or if for any reason the}^ concentrated upon a few fish, they 

 might produce the fatality. The solution of the whole problem lay 

 in the number of the parasites, and ever}' efiort was made to discover 

 a reason for their abnormal abundance as well as that of the other 

 small Crustacea already mentioned. The reason came from a wholly 

 unexpected source; as already stated, a fine lot of larv» were obtained 

 from the eggs laid in aquaria. One of the marked differences between 

 the larva and the adult appears in the anterior maxillipeds. In the 

 larva these terminate in an enormous ])arbed claw instead of a sucking 

 disk. They are, therefore, clasping organs of a highly developed kind, 

 but repeated efforts failed to induce the larva^ to use them for that 

 purpose. 



On the contrary, the latter continued to swarm near the surface of 

 the water on the side of the aquarium toward the light, like other 

 copepods, and paid no attention whatever to several fish which were 

 put in with them as possible hosts. 



Finally two small dace from the same pond were tried, but no more 

 attention was paid to them than to the other kinds. 



Not so with the fish however, for no sooner had thej^ recovered from 

 their fright at being handled than they turned round and ate up every 

 last larva — several hundred in all. 



Inquiries suggested by this action and made as soon as possible 

 revealed the fact that for the three years previous to the fatality 

 among the fish the proprietor of the pond had dragged it thoroughly 

 with a seine and removed all the small dace and roach and sold them 

 for live bait. Here was a satisfactory explanation of the al)normal 

 abundance of copepods and cladocerans, and in it there is a plain sug- 

 gestion that this question of parasitism is not such a one-sided affair 

 as it appears at first sight. The adult Argulus ma}' become a menace 

 to the adult fish, but in its larval stages it no doubt often furnishes 

 food for the young of the same fishes or of others. 



It also suggests that a proper amount of protection for the small 

 fish, such as dace, roach, etc., which inhabit our ponds and streams 

 will be one of the most effective means of guarding against any formi- 

 dable increase in the numbers of these parasites. 



Especially is this true in the case of restricted areas like artificial 

 fish ponds, hatcheries, aquaria, etc. ; here every one of the conditions 

 deleterious to the fish is advantageous to the parasite, and the latter is 



