146 University of California Publications in Zoology [ V OL. 19 



Trypanosoma to show in detail the comparative morphology in this 

 particular stage. C. leptocoridis (fig. A, 1) and C. euryophthalmi 

 (fig. A, 2) have been selected to represent the structure of Crithidia 

 and Trypanosoma leivisi (fig. A, 3, after Minchin and Thomson, 1915, 

 pi. 37, fig. 66), T. triatomae (fig. A, 4), and Schizotrypanum cruzi 

 (fig. A, 5, after Chagas, 1909, pi. 13, fig. 16) that of Trypanosoma. 



Each crithidial flagellate of this series has an elongate body, 

 cylindrical in outline but slightly flattened at the anterior portion, 

 which forms the undulating membrane. At the edge of this membrane 

 there is a sharply defined flagellum of variable length (fig. A, 1, 3). 

 The length of the flagellum has no particular significance since con- 

 siderable variation exists within each species. Posteriorly, however, 

 there is in the stomach phase a consistent difference between the 

 crithidial form of Crithidia and the crithidial form of Trypanosoma. 

 The posterior ends of the bodies of the true crithidias are more atten- 

 uate (fig. A, 1, 2) in the stomach phase, and do not show the slight 

 tendency to become club-shaped until the rectal phase is reached. In 

 figure A, 4, T. triatomae is quite blunt at the posterior end, and the 

 shifting of the cytoplasm into this region is increasing the width of 

 the body at the expense of the length. Another difference almost as 

 consistent as the one just noted is the variation in the nucleus (n.). 

 In Crithidia (fig. A, 1, 2) it is usually anterior to the center, and. in 

 the crithidial forms of Trypanosoma it is posterior to the center (fig. 

 A, 3, 4, 5). 



Internally the similarity of the structure of the organelles and their 

 relationship in general is quite marked. The nucleus (n.), the ble- 

 pharoplast (bl.), and the parabasal body (ph.) are common to each 

 of these flagellates. The nuclear rhizoplast (rh.) and the parabasal 

 rhizoplast (pb. rh.) are found in all the above flagellates with the 

 exception of T. lewisi (fig. A, 3). The absence of these organelles in 

 the crithidial form of T. leivisi is questionable since they are present 

 in all the others. They may have been overlooked because of the 

 delicacy of their structure and the faintness with which they stain. 



The nucleus and the extranuclear organelles will now be taken up 

 in detail. 



Nucleus. This organelle in each of the above flagellates may be 

 described as round or slightly oval in shape and of the vesicular type. 

 It varies somewhat in size from I/A in figure A, 3 to 1.7/* in figure 

 A, 5, but it is usually about two-thirds of the width of the body in 

 diameter. In Crithidia leptocoridis the nucleus shows clearly the ves- 



