198 ILLINOIS BIOLOGICAL MONOGRAPHS [486 



side of them and even among the subcuticular nuclei in the proboscideus form. 

 In toto mounts of anterior segments they may be seen to be quite discontinuous 

 and, as pointed out by Llihe (1900a), arranged in two lateral fields on each 

 surface, there being a few, however, in the median ventral line. The com- 

 bined ootype and shell-gland is a small inconspicuous compact structure lying 

 close to the dorsal wall of the medulla as in A . rugosum. The uterine duct also 

 takes only a very few coils before expanding into the capacious uterus-sac. 

 Matz described the latter as being not round as in B. scorpii and B. claviceps, 

 but pointed towards each side, which points do not disappear when the sac is 

 filled with eggs, while Liihe (1910:17) repeats this statement. In the material 

 studied by the writer only the young uterus-sacs, much elongated in the 

 transverse direction, were found to be pointed laterally, but the mature struc- 

 tures, i.e., when filled with eggs, distinctly rounded or only very broadly pointed 

 in some cases (Fig. 77). In doroso ventral view the sac varies in shape from an 

 elHptical or quadrate, lobed organ, filling up most of the proglottis in the fresh- 

 water form, to a transversely much elongated cavity in the proboscideus form, 

 so enlarged in mature joints that the strobila in such gravid regions is little 

 else than a tube filled with eggs. The openings are situated in the median 

 line on the ventral surface, each one being just opposite the posterior emargin- 

 ation of the segment immediately ahead. 



Matz states that the egg measures 54.5 by 40.9/x, but the writer found 

 them of quite different sizes when removed from gravid segments in the 5% 

 formalin solution in which they were preserved. In general, two sizes were 

 seen, small ones with thicker darker shells and larger ones with thinner lighter 

 shells, but at the same time all intermediate sizes between these forms. Those 

 from the fresh- water form of the species measured 55 to 115 by 35 to 75fj,, while 

 those from the form from Salmo salarwere 45 to 110, by 30 to 75/i, thus showing 

 that so far as the size of the eggs goes, at least these two forms are one and the 

 same species. 



So far as the writer is aware the development of the egg of this species has 

 been studied only by Koelliker (1843:91) and later by Braun (1889:668, etc.) 

 in review. Several writers have described various young plerocercoids. The 

 youngest found by Olsson (1867:53) was only 2mm. in length and had only 

 three segments, a neck and several longitudinal spiral excretory canals, of 

 which two extended to the anterior part of the head. The triangular caudal 

 piece of this young strobila had a median sinus posteriorly, thus indicating, 

 perhaps, in the light of the present contributions, that a portion had already 

 diasppeared. Leuckart (1878:605) spoke of the simplicity of the development 

 of the plerocercoid, while Zschokke (1884:27) believed that he had found the 

 larvae of B. infundibuliformis in numerous cysts on the outside of the walls of 

 the ahmentary tract of Perca fluviatilis, Trutta vulgaris, Esox lucius, Salmo 

 umbla, Thymallus vulgaris and Lota vulgaris. They were also found on the 

 liver, the spleen, the ovaries and the peritoneum of the same fishes, with their 

 scoHces ordinarily invaginated and with lengths of from 2 to 6mm. In 1893 

 Olsson again referred to the plerocercoids and younger strobilas. In Lota vul- 



