ECTOPARASITIC TEEMATODES. 275 



was not well founded. While I was working in Mr. Agassiz's 

 laboratory in Newport I collected some specimens of Octocotyle 

 from the gills of the mackerel {Scomber scomber), and an exam- 

 ination of these has shown that they belong to my 0. major, 

 having five pairs of penis hooks and a single pair of hooks at 

 the posterior end of the body, the forms of these hooks as well as 

 the other characters perfectly agreeing with those of 0. major. 

 This has revived the suspicion expressed in my " Studies," and 

 a comparison of the figures and descriptions of Dujardin, van 

 Beneden, and Parona and Perugia has led me to reverse my 

 former conclusion and to recognize two species in the published 

 descriptions of 0. scombri. Thus, Dujardin as well as Parona 

 and Perugia give the number of penis hooks as twelve ; again, 

 according to Parona and Perugia ['89] there are two pairs of 

 hooks at the posterior extremity of the body, of which the in- 

 ternal pair are considerably smaller than the other. The figures 

 of these hooks given by thé last named authors are, indeed, 

 totally different from those of 0. minor, and I was led by them 

 to the conclusion stated in my " Studies ;" but a careful study 

 of the description of Parona and Perugia leads me to infer that 

 the words " fig. 3," referring to the hooks in question, must be 

 a la2:)8us calami, and that the figures must represent the penis 

 hooks. Mark, on this point, the words describing these hooks, 

 " quelli del paio superiore sono piu esterni, piu grossi e con due 

 denticoli basali.'' It appears, then, that Dujardin and Parona and 

 Perugia had before them 0. minor. It must be remarked, however, 

 that the dimensions given by Dujardin correspond better to those 

 of 0. major. Van Beneden and Hesse, on the other hand, give the 

 number of penis hooks as ten, which is characteristic of 0. 

 major. 



