[121] NOTES ON ENTOZOA OF MARINE FISHES. 839 



32. Rhynchobothrium heterospinc, sp. nov. 

 [Plate xn, Figs. 3-6.] 



On August 4, 1886, I obtained a single immature Rhynchobothrium 

 from the spiral valve of the smooth dog-fish (Mustelus canis), which, 

 upon re-examiniug at leisure, I find is specifically different from the 

 other Rhynchobothria in my collection. I either neglected to make notes 

 of this specimeu while it was living or, if notes were made, they have 

 not since turned up. The specimen has been subjected to some press- 

 ure while in a fresh state, and its flattened condition makes it difficult 

 to determine whether the bothria are marginal or lateral, and, of coarse, 

 exaggerates the measurements of breadth. 



Bothria two, opposite, lateral, short-elliptical or suborbicular. Neck 

 long, cylindrical, compressed, slightly enlarged at base, in vicinity of 

 contractile bulbs. Proboscides very long and slender, sheaths spiral, 

 contractile bulbs linear-oblong. Hooks mostly slender, but of very 

 diverse shapes. * 



Segments begin some distance back of contractile bulbs. Last seg- 

 ments elongate. Genital apertures marginal. 



Habitat. — Mustelus canis, spiral valve, single immature specimen, Au- 

 gust 4, 1886, Wood's Holl, Massachusetts. 



The dimensions of the alcoholic specimen, much flattened, are as fol- 

 lows: Length, 13.50 mm ; length of head, 0.60 mm ; breadth, 0.70 mm ; di- 

 ameter of neck, 0.26 mm ; length of head and neck, 3.00 mm ; length of 

 proboscides, approximate, 1.60 mm ; length of contractile bulbs, 0.52 mm ; 

 breadth, 0.12 mm . 



In this specimen, which is evidently immature, the segments are in- 

 distinct. The last segment measures 1.80 n,m in length and 0.50 ram in 

 breadth and tapers to a blunt point at the posterior end. Three or four 

 elongated segments are marked off at the posterior end of the strobile by 

 faint transverse lines. These are filled by the characteristic nuclear 

 masses which precede the genitalia in the Cestoda, and from which the 

 organs of the segment are differentiated. The genital organs are not 

 yet distinct but are sufficiently developed to show that the male genital 

 aperture is marginal. 



The diameter of the proboscides, exclusive of hooks, is about 0.03 mm ; 

 with hooks included it is about 0.05 mm . The maximum length of hooks 

 is 0.027 mm , their breadth 0.005 mm . In general the hooks are of at least 

 four kinds, which, graduating as they do into each other, produce some 

 very diverse forms. First, there are nearly straight hooks tapering to 

 a sharp point ; second, slender, arcuate, sharp-pointed hooks; third, 

 slender, straightish hooks with abruptly recurved point, some transition 

 forms with gentle sigmoid curve like the leter S partly straightened out; 

 fourth, like the third form, into which it graduates, except that the shaft 

 of the hook is quite broad in a plane coinciding with the longitudinal 

 axis of the proboscis, the recurved end separated from the shaft by a 



