84 



ILLINOIS BIOLOGICAL MONOGRAPHS 



[372 



Olsson spoke of the larva of T. rohistus being provided with a narrower cyhn- 

 drical "cauda" as in certain Tetrarhynchus larvae, and gave the length of one 

 as 120nim., while the anterior portion was approximately 60mm. long. Such 

 structures were seen posteriorly in many of the larger larvae at hand but their 

 lengths and degrees of distinction from the fore-body varied considerably. 

 A medium large one, in a lot from Esox lucius, for instance, had these measure- 

 ments: Length of anterior portion, 48mm., of cauda 24, of scolex 1.16; width 

 of forebody 0.66, of cauda 0.37 (3:1 — Olsson). The specimen from Leucich- 

 thys artedi dealt with in the third column of the above table possessed a cauda 

 110mm. in length, while that treated in the fourth column had no posterior 

 appendage at all. As for its anatomical structure this organ is characterized 

 by very poorly developed cuticula and musculature, and a ver\' loose or open 

 parenchymatous mesh-work. 



As may be seen by reference to the host record given below the specimens 

 taken from Leucichthys artedi were the only ones belonging to this type of 

 larva which were found encysted. From 26 specimens of the host examined 

 by the writer, 14 cysts, each containing a single larva, were taken. Each fish 

 harbored one or two cysts, but one contained four. The cysts themselves 

 are from 15 to 45mm. in length and 2 to 3mm. in diameter, and cyUndrical in 

 shape, with bluntly pointed ends. They are often attentuated at one end to 

 such an extent that at first sight they appear to be terminated by a mere thread. 

 This is found, however, on closer examination to be hollow and to place a 

 more distal, but som.ewhat smaller portion of the cyst in communication with 

 the main body. In situ these cysts are located constantly in the myocommata 

 of the dorsal musculature of the host, from a short distance back of the dorsal 

 fin to close behind the skull, and with* their longitudinal axes directed down- 

 ward, inward, and forward towards the spinal column, or, indeed as was seen 

 in one case, actually towards the dorsal aorta. Often they are found doubled 

 on themselves in a V-shaped manner. Their translucent white or somewhat 

 opalescent color is due to the contents, which besides the worm coiled on itself 

 several times consists of a caseous substance, showing thru the thin wall. 



On the other hand, the other type of larva which is considered by the 

 writer to belong to T. nodulosus, is characterized by a much shorter, narrower 

 body, smaller at irregular intervals owing to differences of contraction, whence 

 the specific name, and by a quite different scolex provided with the well- 



