465] LARVAL TREMATODES—CORT 19 



begun to spread out a little forward and to the sides, but for the most 

 part backward along two irregular lines. At first all the pigment 

 strands were connected in an irregular way, but soon at the ends of 

 the lines little flecks were scattered out. Backward along the lines 

 the pigment becomes thicker (Fig. 20, p) and collects at points of union 

 in the network masses. At an older stage, (Fig. 21, p) the pigment 

 strands from the two lateral lines become connected and form an irreg- 

 ular network about the middle of the oral sucker, laterad as far as the 

 sides, and backward to about the middle of the body. As the cercariae 

 become older the masses and lines of pigment break up and are more 

 scattered. In the mature specimen (Fig. 16) the pigmentation shows 

 as scattered brownish flecks extending thru the whole thickness of the 

 body (Fig. 17, p) and reaching from the oral sucker back to the middle. 



The cystogenous glands in Cercaria inhabilis are as thickly devel- 

 oped dorsally as ventrally, and extend from the oral sucker to the 

 acetabulum. Viewed from the surface they appear as small rounded 

 bodies, 0.012 to 0.016 mm. in diameter, filled with rod-shaped cysto- 

 genous granules. Figure 17 shows them to be elongate, unicellular, club- 

 shaped glands with small nuclei. 



The tail of Cercaria inhabilis varies from one-third to greater than 

 the body length. It is attached to the tip of the body above the acetab- 

 ulum and is easly lost in free swimming animals. The cuticula of the 

 tail is very thin and no trace could be found of circular muscles. The 

 longitudinal muscle layer, however, is conspicuous and is formed of a 

 series of strands each 0.0035 mm. in thickness, which extend from the 

 base to the tip. Inside of the muscles is a layer of irregular paren- 

 chymatous cells, with nuclei 0.005 to 0.007 mm. in diameter, and irreg- 

 ular indistinct cell boundaries. The space between this layer and the 

 excretory tubule, which courses down the center of the tail, is filled 

 with large cells with faintly granular cytoplasm, and large nuclei, 0,008 

 to 0.009 mm. in diameter. These cells are similar to those forming the 

 core of the tail of Cercaria urbanensis, but inclose no such darkly stain- 

 ing granules. Several of these cells may occur in one cross section since 

 they do not seem to be arranged in regular rows. The central excretory 

 tubule of the tail has a considerable diameter, and contains in its walls 

 very large scattered nuclei, 0.01 to 0.012 mm. in diameter. A cross 

 section of the tail (Fig. 22) shows these structures. 



A comparison of the nucleoli of the different nuclei of the tail of 

 Cercaria inhabilis is interesting. Of the three kinds of nuclei present 

 viz. 1. nuclei of the excretory tubule, 2. nuclei of the large central 

 cells, 3. parenchymatous nuclei, the first two have large very clearly 

 defined nucleoli and little if any chromatin scattered outside of it. Of 



