MUSEUM OF COMPARATIVE ZOOLOGY. 31 



of the adult spermatozoon is reached. Many stages of development can 

 be seen in the same testis. The ditiereut stages occur in distinct groups, 

 each group probably being the product of one of the parent cells. The 

 wall of the testis, when the spermatozoa first begin to develop, is com- 

 posed of many cells, most of which by division go to form spermatozoa ; 

 a few of the cells, however, are differentiated into flattened epithelium, 

 which constitutes the wall of the capsular testis (Plate II. Fig. 24, eth.). 



I have not succeeded in ascertaining the exact manner in which the 

 spermatozoa find their way into the vasa deferentia, but lijima's state- 

 ment ('84, p. 408) that they do not wander through the spaces of the 

 mesenchyma is certainly incorrect. The testes give rise to tubular 

 prolongations, but whether these are directly connected with the vas 

 deferens or first unite into one or more vasa efferentia, I cannot say. 

 The testicular canals appear to be direct outgrowths of the wall of 

 the testis. Their walls and those of the vasa deferentia have the same 

 simple structure (compare Plate II. Figs. 23 and 24), being composed 

 of a single layer of thin epithelium. The nuclei in the walls of the 

 tubes often occur in pairs, and thus suggest that the cells to which they 

 belong have recently undergone division (Fig. 24). 



According to Moseley ('74, p. 139), the testes in land planarians open 

 directly into the vasa deferentia ; Minot ('77, p. 432), on the contrary, 

 speaks of fine testicular canals that unite to form larger tubes. Kennel 

 ('79, p. 137) states that the testes, arranged in rows, fuse to form the 

 vasa deferentia. 



The anterior ends of the vasa deferentia in Phagocata lie on either 

 side of the pharyngeal chamber in the region of the mouth opening. 

 They have the form of large elongated sacs (Plate IV. Fig. 42, x) which 

 open into comparatively narrow tubes {va. df.), which are of an even 

 calibre, and much convoluted and twisted. They run backward parallel 

 to each other until near the base of the penis ; they then turn at right 

 angles toward the middle plane, where they unite to form a single tube 

 which terminates at the apex of the penis. The spermatozoa when ripe 

 leave the testes by the testicular canals previously described, and pass 

 into the vasa deferentia, which become filled from their enlarged blind 

 ends up to a point beyond that where they unite to enter the penis. 

 Here the spermatozoa remain stored until arranged into spermatophores, 

 in which form they pass into the vagina of another individual. After 

 the spermatozoa have found their way to the vasa deferentia, all traces 

 of the testes disappear. 



Physiologically considered, the vasa deferentia of Triclads are to be 



