INTERNAI. PARASITES OF THE SEBAGO SALMON. 1 1 77 



centers of the two suckers was 0.5 mm. In one 10 mm. long this distance 

 measured i mm. 



The posterior region is nearly a perfect cylinder until shortly before the 

 tip, where it tapers somewhat. In some specimens the posterior end is consid- 

 erably inflated and appears semitranslucent. This is undoubtedly due to the 

 distended condition of the excretory reservoir, which inhibits contraction of 

 the circular muscles in the portion of the skin adjacent to it. 



The breadth of the body varies according to the degree of contraction, but 

 may be estimated in general as from 0.7 to i mm. An immature specimen 

 2.85 mm. long measured 0.65 mm. in breadth between the suckers, 0.6 mm. 

 behind the acetabulum, and 0.52 mm. behind the posterior testis. An imma- 

 ture specimen only 1.6 mm. in length measured 0.32, 0.28, and 0.21 mm. in 

 breadth at the same points. In cross section the body is round or very 

 slightly oval. 



The oral sucker is subterminal and its opening looks almost directly ventrad. 

 It is rather conspicuous, and in an average specimen measured 0.68 mm. in 

 antero-posterior diameter and 0.67 mm. transversely. The depth in the same 

 specimen was 0.6 mm. The orifice is nearly circular, though often appearing 

 slightly flattened along the posterior margin. In an immature specimen 2.85 

 mm. long the oral sucker measured 0.35 mm. in antero-posterior diameter and 

 0.4 mm. laterally; the orifice measured 80 by 150 yu. 



The ventral sucker or acetabulum is usually distinctly smaller than the 

 oral. In the extreme case it appears about equal in size or, on the other hand, 

 only about half as large. Ordinarily it is prominent, but in short, thick specimens 

 it is almost hidden, whereas in elongated, slender specimens it projects so far as 

 to appear almost pedunculate. It is also often slightly oval in a transverse 

 plane. In an adult specimen it measured 0.57 mm. in antero-posterior diameter 

 and 0.69 mm. laterally. In a specimen 2.65 mm. long the corresponding meas- 

 urements were 0.3 and 0.33 mm., and the orifice measured 52 by 80 fi. 



The alimentary canal opens in the oral sucker, close behind which lies the 

 pharynx without any prepharynx between the two. The pharynx measures 

 0.21 by 0.13 mm. It is often seen in the vertical position represented in the 

 figure of Messrs. Anderson and Boyden, which I have taken the liberty of copying 

 here. The oesophagus is very short and it often proceeds anteriad from the 

 upright pharynx, as shown in the drawing (fig. 3, pi. cxxi). At its tip start the 

 two branches of the intestine, which also usually extend forward a short distance 

 and then turning posteriad continue almost to the extreme posterior tip of the 

 body. These crura being longer than the body in the usual specimen are thrown 

 into folds, which often appear as if the canal possessed irregular outpocketings, 

 such as one finds in Paragon imus. Observations both on the living material and 



B. B. F. i9oS-Pt 2—32 



