158 BULLETIN OF THE BUREAU OF FISHERIES. 



There is no reason for believing that larger numbers of fish would present any more 

 serious difficulties than are to be expected in the keeping of any lish in large numbers 

 within a restricted space; and, if one could insure as uniform and careful an infection of 

 the larger numbers, we have every reason to believe that such infections would prove 

 as successful as those here described. 



INFECTIONS WITH HOOKED GLOCHIDIA. 



For the infections with hooked glochidia, we have used principally Anodonta cata- 

 ractairom Falmouth, Mass., the species studied by Lillie (1895). With these we infected 

 German carp under 6 inches in length and, unless otherwise stated, the following account 

 refers to this combination which gives typical results. A smaller number of infections, 

 made with Symphynota complanata and S. coslata upon carp and other fishes, are referred 

 to in a supplementary manner. The glochidia of .4 . caiaracta become attached in large 

 numbers to the fins (fig. 19-25, pi. ix and x) and gills of the carp. They are also found 

 upon the other external parts which offer the condition of a soft scaleless epithelium like 

 that of the fins; thus, the region about the anus, the edge of the operculum, the lips and 

 in very heavy infections, even the soft area of the ventral surface between the mouth and 

 pectoral fins may become heavily loaded. Within the mouth cavity, the gill filaments 

 and also the gill bars and rakers become well covered. " The glochidia which attach to 

 these mouth parts do not remain, for, although the fish may be carrying many of their 

 fellows upon its external parts, in about one week after the infection all glochidia have 

 disappeared from the gill filaments, which then become as clean as though never infected. 

 Scattered glochidia may remain upon the other internal mouth parts, for specimens are 

 occasionally seen well embedded and in advanced stages of their metamorphosis, but in 

 the main these parts also will become free of glochidia. 



The general distribution upon the individual fins may be seen by reference to figures 

 19 to 25, plates IX and x, which show how great a proportion of the glochidia become 

 attached to the fin margins. If a fish is carefully watched, as its slight movements stir 

 up the glochidia during the infection, the latter are seen continually falling upon the 

 upper faces of the pectoral and pelvic fins. They may even be collected with a pipette 

 and heaped upon a motionless pectoral fin, remaining there for some minutes without 

 more than an occasional specimen becoming attached. The margin of the fin is so much 

 more favorable for attachment, that it is often thickly set with glochidia, when none are 

 found upon the fin surface, and this despite the fact that glochidia must, during infection, 

 strike against the surface of the fin many times for every time that one of them comes 

 in contact with a fin margin. It is, therefore, the margin of the fin for which this glochi- 

 dium is best suited, and, once fastened there, it is almost certain to remain and become 

 embedded by the growth of the host's epithelium. 



Considered in a more detailed way and with reference to the parts of the glochidium, 

 we may explain this more frequent attachment to the margin as due to the fact that 

 when the glochidium strikes against any flat surface the sensory hairs are not stimulated 

 and the glochidium, which, as we have already shown in the case of the hooked forms, 



