488 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvin. 



Many species show a tendency to congregate in certain places to the 

 almost entire exclusion of the rest of the fish's body, as in the case of 

 Caligus bonito and Zepepphtheirus pectoralis. 



While many of the species stick to one particular host there are 

 others which change hosts from time to time and which are able like 

 the Argulida? to live temporarily upon almost any fish that may be 

 available. Caligus rwpax easily takes the lead in this, having been 

 found upon twenty-five or thirty different fish. A few of these like 

 the flounder and cod are regular hosts, and an examination of a very 

 few fish is almost certain to reveal the presence of this parasite. 

 At the right seasons also the chalimus stages of development may be 

 found attached to the fins and scales of the same fish. But for the 

 other hosts there is often a record of only a single specimen which was 

 evidently a straggler and took that particular fish until it could find 

 something better. 



FOOD. 



These parasites feed upon the blood of their hosts which th&y obtain 

 in the same manner as did the Argulids by burrowing under the scales 

 or piercing the skin on the fins with their maxillipeds and proboscis. 

 This blood, filling the central digestive system, may often be seen as a 

 dark streak through the body, and is sometimes very prominent in 

 transparent specimens. 



When taken from the fish those specimens usually live the longest 

 which have the most blood in them; the latter seems to digest slowly 

 and may often be seen for several da} 7 s in the intestine. 



Many authors write that these parasites, or some of them at least, 

 feed upon the mucus of the fish's body and that no blood has ever been 

 found in their stomachs. 



This statement seems to rest entirely upon the fact that no red color 



can be seen in their digestive organs. A. Scott sa} 7 s of Lepeoplitheirus 



pectoralis, " when taken directty from the living fish and placed under 



the microscope it rarely shows even the faintest trace of red coloring 



.matter in the alimentary canal." 



But the same author concludes on the next page that this species 

 feeds to some extent on blood, and a little later he adds "they do not 

 hesitate to eat their comrades when these become feeble." Far animals 

 which will do this mucus must be a rather tame and inadequate food. 



It is difficult to determine what the food realty is, but there are sev- 

 eral considerations which will help us to form a rational judgment in 

 the matter. In the first place, it makes a difference what part of the 

 fish's body the parasite is taken from, whether it shows any red in the 

 digestive organs or not. 



If taken from the gill cavity the red is nearly alwaj^s prominent, 

 while it seldom appears in those individuals taken from the outside of 

 the body. Some species are always found in the gill cavity and they 



