no. 140-t. PARASITIC COPEPODS—CALIGIDM-WILSON. 489 



always show blood in the alimentary canal. Other species are always 

 found on the outside of the body and they are the so-called mucus 

 eaters. But then 1 are still other species, like Caligus ra/pax, which may 

 be found in either place, and in them we find the same difference. 



In explanation we must remember two facts: The gill cavity is the 

 easiest place to get blood on the fish's body, and it is very possible 

 that such a species as O. rapax may slip into this cavity to get its 

 food and then slip out again to the exterior of the body. Consequently 

 when taken in the gill cavity it would have just finished eating, while 

 on the outer surface of the body the blood may have had time to par- 

 tially digest. 



The second fact is that all these genera are supplied with powerful 

 digestive glands unlike the Lerneans. We shall see later (p. 513) that 

 one pair of these glands are situated in the anterior part of the cara- 

 pace and pour their fluid upon the food as soon as it strikes the stom- 

 ach. Only freshly aerated blood, in or near the fish's gills, has a deep 

 red color; that in the capillaries of the skin and fins is not very red 

 when swallowed. Hence it would not take very much of a digestive 

 fluid to remove the color entirely. 



It is very suggestive to note in this connection that the adult Ler- 

 neans, in which the food is so red as to leave no doubt of its nature, 

 have no digestive glands. During development, however, there is a 

 digestive gland, and the contents of the alimentary canal are not reel. 

 It is scarcely possible that the young Lernean eats mucus while the 

 adult eats blood. 



Again, if these creatures are seeking mucus for food why do they 

 not choose such fish as are most bountifully supplied with it? And 

 how does it happen that they always hunt out those places upon the 

 fish's body where the skin is unprotected and the thinnest? There is 

 more mucus on the scales than anywhere else; why should they choose 

 the fins or the inside of the operculum? Finalh 7 , the mouth parts of 

 species, which it is claimed cat mucus, are similar in every particular 

 to those of species which are acknowledged blood-suckers. The 

 eating of mucus, then, must be an acquired taste, and it is difficult to 

 understand how an animal with its mouth parts fitted for sucking 

 blood should be content to merely sip mucus while it still retains 

 enough of its old blood-sucking habits to choose the places on the fish's 

 body best adapted for that purpose. 



These parasites are very susceptible to any increase in temperature, 

 and a rise of a few degrees will speedily prove fatal. 



This is the chief source of difficulty in keeping them alive in aquaria. 

 A. Scott gives L6 ('. as the limit for Lepeophtheirus pectoralis, and 

 experiment has shown that other species do not differ much from this. 

 A general average of all the species experimented with would raise 

 this limit slightly, to 18° or 20^ C. 



