MUSEUM OF COMPARATIVE ZOOLOGY. 35 



The absence of yolk glands in Moseley's land planarlans can be 

 accounted for by assuming that in his material they were not yet ripe, 

 as was probably the case. He states ('74, p. 137), however, that there 

 is occasionally present in Bipalium, "just externally to the lower ex- 

 tremities of the ovaries, a small mass of large nucleated cells connected 

 by a pedicle with the ovary itself." He considers that " it may repre- 

 sent a yolk-gland in a rudimentary condition." With this 1 fully agree, 

 and further believe that this rudimentary yolk gland is the homologue 

 of the structure which in Phagocata I have called parovarium. 



The presence of a vitellogenous organ in Phagocata, together with 

 the condition found in Polycelis by lijima and in Bipalium by Moseley, 

 suggests a discussion of the relations of the ovaries and vitellaria. Yolk 

 glands have long been considered as resulting from the differentiation of 

 the ovaries. Gegenbaur, as stated in his text-book ('70, p. 281), con- 

 siders the yolk glands to be " Theile eines ansehnliches Ovars." Hallez 

 ('79, p. 63) maintains that " le viteUogene n'est autre chose qu'une partie 

 diflferenciee de I'ovaire," and according to Lang ('81*, p. 228), " Die 

 Keimstocke und Dotterstocke der Tricladen sind einander gleichwerthig. 

 Sie entstehen aus Zellen, die anfangs nicht von einander unterscheiden 

 lassen." Among Rhabdocoeles all gradations are found, from an undif- 

 ferentiated " Keimdotterstock," where ova and yolk cells are developed in 

 different portions of the same organ, to conditions in which the ova and 

 yolk cells are produced in distinct and separate organs. The yolk glands, 

 then, have arisen by a divison of labor from a simple germ gland, as has 

 already been formulated by Graff ('82, p. 130) in the following words : 

 ** Die Keimdotterstocke mussen wir uns aus Ovarien durch einfache 

 Arbeitstheilung hervorgegangen denken ; durch raumliche Trennung 

 der verschieden functionirenden Abschnitte des Keimdotterstockes ent- 

 standen schlieslich die Keim- und Dotterstocke." I consider the condi- 

 tion found, in Phagocata to be less differentiated than that exhibited by 

 PI. tenuis (lijima), inasmuch as the cells which form it still retain a moi-e 

 intimate relation to the true ovary than they do in the latter case. 



The union of the yolk glands with the oviducts is a secondary one ; it 

 takes place at intervals throughout their length. I have not studied 

 this in detail, but, as far as I have learned, the conditions agi-ee with 

 the careful description given by lijima ('84, p. 41-5). The oviducts 

 open into the vagina just above thq point where it enters the genital 

 atrium (Plate IV. Fig. 42). 



The uterus (Plate IV. Fig. 42, ut.) is a sac-like organ lying just 

 anterior to the penis, and has thick walls that are thrown into many 



