176 University of California Publications in Zoology [VOL. 19 



becomes torn very readily ; it has every appearance of having been 

 almost entirely destroyed by the crithidias. Among these small hapto- 

 monads numerous variations of size and shape are noted. The length 

 of the free flagellum (pi. 6, figs. 109, 110) is exceedingly variable. 

 In other crithidias whatever flagella are present are intracellular 

 throughout their length. The changes brought about in the flagella 

 are probably due to their absorption. In every case the flagellum 

 attaches the crithidia to the wall of the digestive tract. The nuclear 

 structure of the haptomonads in this region, with the exception of 

 the forms in figure 91, shows no indication of degeneration. The round 

 haptomonads finally become free forms. They can be seen to drop 

 off in the living preparations and to roll up into round or oval forms 

 (pi. 6, figs. 118, 119, 122). The cytoplasm of these forms is vacuolate 

 and stains lightly. These round crithidias then degenerate along with 

 the nectomonads, which are constantly degenerating. The degenera- 

 tion of the haptomonads and nectomonads will be described under 

 the degenerative series. 



FINAL SPOEE FORMS 



The structure of the digestive tract of EuryophtJialmus co-nvivus, 

 including three portions of the stomach proper and the intestine with 

 its gland, differentiates very clearly between the degenerative series 

 of crithidias and the final spore forms which can be transmitted to 

 another host. In the anterior part of the digestive tract is found the 

 developmental series which becomes the degenerative series together 

 with the final spore forms. Posterior to the gland only the final 

 spore forms have been observed. The preparations of the rectum 

 show only these final spore forms (pi. 6, figs. 125-131). These are 

 oval, non-flagellated forms containing a thick periplast, within which 

 are the nucleus and parabasal body. In figure 126 the characteris- 

 tics of these final spore forms may be noted. They are approximately 

 2.8/>t in length and 1.4ft in width. The nucleus lies in the extreme 

 posterior end of the body and stains deeply. The parabasal body is 

 sharply outlined within a vacuolate area. A faint nuclear rhizoplast 

 may be visible, extending from the nucleus toward the parabasal 

 body. As previously indicated in a preliminary paper (McCulloch, 

 1917) it was some time before the true rectum was discovered, and 

 consequently the significance of these final spore forms was not en- 

 tirely clear in the beginning of the investigation. However, with the 

 discovery of the true rectum and the fact that it contained only these 



