STRUCTURE OF CLINOSTOMUM 219 



lines parallel with the surface from which the fibers of the paren- 

 chyma radiate. A sharp line bounds this mass of specialized 

 parenchyma which thus constitutes a capsule enclosing the uter- 

 ine sack. The fact that the uterus enters ,the sack in the center 

 of its dorsal surface and not at the anterior end prevents us from 

 regarding the sack as merely a dilatation of the uterus. We must 

 however think of it as having arisen as a differentiation which has 

 taken place in a loop of the uterus. In most of the species of this 

 genus (Braun, '00) there is a similar blind sack into which the 

 uterus enters and which extends blindly in front of the end of the 

 uterus. In one species however, (C. heterostomum, Braun, '00, 

 fig. 1), the uterus passes forward to the posterior border of the 

 ventral sucker where it bends and runs straight back again to end 

 at the female genital opening. This is doubtless the more prim- 

 itive anatomical arrangement, and the one from which the sack 

 form has been developed. We note also in passing that this 

 species is more primitive, too, in possessing well developed diver- 

 ticula of the intestinal caeca. 



The form of the uterine sack in the D. reticulatum of Looss, 

 as described and figured in his paper ('85), is decidedly different 

 from that which I have just described. The sack in that species 

 is elongated posteriorly to reach a point posterior to the posterior 

 testis (fig. 22) . In fig. 26 we learn that the part of the sack which 

 leads to the exterior is a lateral offset from the main sack. This 

 posterior portion of the sack of Looss is wholly wanting in my 

 material. Cross sections (e.g., fig. 4 F) show that the sack does 

 not extend into the testis region of the body. This is an inter- 

 esting point. It does not seem possible to doubt the fact as 

 related by Looss for his form. In every other respect the form 

 D. reticulatum bears the closest resemblance in organization to 

 C. marginatum, and writers from Leuckart down have considered 

 them identical. Thus Leuckart ('89, p. 401) says D. reticula- 

 tum" Mit Leidy's Clinostomum gracile zusammenfallt." Stiles 

 and Hassall ('98) say "Looss described as Distomum reticulatum 

 a form which is evidently identical with Leidy's Clinostomum as 

 Leuckart has already surmised," etc. And MacCallum ('99, p. 

 705). The description given by Looss of D. reticulatum 



. . . applies so exactly in every particular to the forms we 



