22 PROCEEDINGS OF THE NATIONAL, MUSEUM VOL. te. 



PARORCHIS AVITUS Linton 



This distome, from the cloaca of the herring gull {Larus argenta- 

 tus), was reported in 1912.« (Cat. No. 7942, U.S.N.M.) 



Prof. William Nicoll has written me suggesting that this is the 

 same as Zeugo^^chis acanthus (renamed by Nicoll Parorchis acan- 

 thus)^ found by him in the bursa Fabricius and cloaca of the herring 

 gull.* I hesitate, however, at present to make any change in my classi- 

 fication for the following reasons: 



Braun in 1901 gave the name Distomum pittaciuin to a distome^ 

 represented by a single specimen from Tringa interpresJ' The 

 main difference between D. pittacmm^ on the one hand, and P. aoam- 

 thias and P. avitus on the other, is the absence of spines and of a 

 circumoral collar. The absence of spines is an unimportant differ- 

 ence, as they may be more or less evanescent. The absence of a cir- 

 cumoral collar is harder to account for, but, since Braun had only 

 the one specimen, and that possibly not in the best state of preserva- 

 tion, it is certain, in view of the very close resemblance in details 

 of anatomy in these three differently named distomes, that they are 

 very closely related, if not identical species. 



Nicoll notes the remarkable resemblance between D. pittacium and 

 P. acar\,thias^ but indicates certain points in which they differ. Thus 

 the ratio of the diameter of the oral sucker to the ventral surker in 

 D, pittaciwni is 1:3, while in P. acanthias it is nearly 1:2. In P. 

 acanthias the pharynx is slightly larger and the testes much smaller 

 than in D. pittaciwm. The convolutions of the uterus are much more 

 extensive in D. pittacium than they are in P. acam,thias, extending, 

 as they do, to the lateral margins of the body, as well as anteriorly 

 to about the level of the middle of the ventral sucker, and posteriorly 

 to a level behind the testes. 



A reexamination of four specimens mounted in balsam shows that 

 with respect to the uterus P. avitus is in almost exact agreement wih 

 D. pittacium. In all of them the convolutions of the uterus extend 

 to the lateral margins. In all of them, also, convolutions lie on each 

 side of the ventral sucker from about its posterior fourth to the pos- 

 terior third of its length. In two of the four specimens convolutions 

 of the uterus extend back to the middle of the testes, in one they 

 extend to the posterior end of each testis, and in one they extend a 

 little way back of the testes, as they do in the type specimen. 



Measurements of four specimejis mounted in balsam, all more or 

 less compressed. 



» Proc. U. S. Nat. Mus., vol. 46, pp. -ool-SSS. 



*Ann. Mag. Nat. Hist., vol. 17, pp. 519-522, flgs. 4-7. 



t Zool. Jahrb., 1902, p. 146, fig. 89. 



