2 I 



Amongst the cells are a few scattered, elongated gland cells, which stand out very prominently 

 in haematoxylin preparations owing to their contents taking the stain very deeply. No nematocysts 

 occur in this portion of the filament, but in its lower layer, and resting upon the mesogloea, are 

 a few muscle fibres, over which are indistinct indications of what is, presumeably, a nerve layer. 



On either side of this median streak is a distal lateral streak {DL), whose cells are 

 somewhat longer than those of the median streak, so that the latter occupies the floor of a 

 rounded groove betvveen the two distal lateral streaks. A close examination shows marked 

 differences in the histological characters of these streaks compared with what was found in the 

 median streak. Thus one notices that there is no distinct layer of cilia on the free surfaces of 

 the streaks and no well-marked cuticular layer-, the gland cells are very much more numerous 

 and nematocysts are abundant. The cells of these streaks rest upon the prolongation of the distal 

 angles of the mesogloeal enlargements and no muscle cells could be found in their lower layers. 



Finally, resting on the lateral surfaces of the mesogloeal enlargement and on the lateral 

 faces of the distal and proximal prolongations of its angles are the proximal lateral streaks {PP), 

 which are composed entirely of slender cells, whose nuclei are near the free surface and which 

 bear strong cilia. The cells which immediately succeed the distal lateral streaks are as high as 

 these, but they gradually diminish in height as they are traced proximally, until those lining 

 the proximal angular processes of the mesogloeal enlargement are very much lower. In conse- 

 quence of this the streaks have a U-shaped form in section, the rounded lower portion of the 

 U resting on the lateral surface of the mesogloeal enlargement, while the uprights rest upon 

 the distal and proximal angular processes. No glands, nematocysts, or muscle cells occur in 

 these streaks. 



It is to be noticed that the endoderm of the mesentery immediately adjoining the proximal 

 lateral streaks is composed of an epithelium whose scattered spherical nuclei are situated near 

 the free surface, the basal portion being clear and having a somewhat reticular appearance. 



In C. americanus the structure of the corresponding portions of the filaments is essentially 

 the same, the only differences to be observed being a greater breadth of the median streak, 

 a slightly better development of its muscle cells and a more spherical or oval form tor its 

 nuclei and those of the proximal lateral streaks. 



With regard to the structure of the craspedonemes, it is clear that they are really folds 

 of the edge of the mesentery over which the filament passes, so that an ascending ancl a 

 descending portion of it may be recognized as the Hertwigs pointed out. In P . fimbriatus 

 they are in all cases comparatively broad ancl low, so that the ascending and descending limbs 

 are separated by a rather broad band of endoderm (fig. 1 3). The proximal lateral streaks are 

 always well defined in them, but some difference is to be found in the structure of the distal 

 streaks as the region where the trilobed portion of the filament passes over into the simple 

 portion is approached. In a section of a craspodoneme of a macrocneme, in which the trilobed 

 portion of the filament extends almost the entire length of the mesentery, the structure ot 

 the limbs is exactly the same as in the filament proper, a median and two pairs of lateral 

 streaks being clearly discernible. But if a section be taken of a craspodoneme from one of the 

 brachycnemes, in which they occur only immediately above the region where the single-lobed 



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