[19] NOTES ON ENTOZOA OF MARINE FISHES. 737 



These differences can not be accounted for by supposing different 

 states of contraction, although it is true that contraction can and does 

 give rise to differences in shape as well as in size. In this case, how- 

 ever, the differences are so profound, and, what is of still greater im- 

 portance, so abrupt, there being no gradation by intermediate forms. 

 I have felt myself obliged to recognize it by establishing two varieties. 

 The specimen with the smaller head and narrower anterior segments I 

 shall denote as variety a, the other as variety /?. One might indeed be 

 justified in separating them yet further and calling them distinct spe- 

 cies if the same sharp distinction is observed in other collections. In 

 that case variety a should retain the name D. microcephalum, and va- 

 riety /3 should be referred to Leuck art's I), sagittatum. 



Both Leuckart and Rudolphi mention the occurrence of individuals, 

 some of which had relatively large, others relatively small heads. Iu 

 the specimens which these observers examined, however, this difference 

 could be accounted for apparently by a difference in the age of the spec- 

 imens. The younger and immature individuals had relatively larger 

 heads and longer anterior segments thaii the more mature specimens 

 possessed. 



I have recorded a similar difference iu a lot of Dibothria from the file- 

 fish (Alutera schcepfii) (U. S. Fish Commission Report, 1886, pp. 458, 

 459, PI. I, Figs. 5-8). In that case both varieties were equally imma- 

 ture. In the present iustauce both varieties are equally mature. 



While there is, therefore, almost sufficient grounds for establishing a 

 new species, or rather for separating the present species into two and 

 restoring Leuckart's species, which has been united with D. microcepha- 

 lia, I shall for the present be content with referring both kinds to I). 

 microceplialum, but shall distinguish the kinds as var. a and var. /?, re- 

 spectively. Wagener's figure, which is sketched from a young speci- 

 men and gives a lateral view of the head and first segments, bears a 

 very close resemblance to var. a. Leuckart's figure also represents a 

 lateral view of the head and anterior segments. It bears little resem- 

 blance to either variety, but resembles var. fi more than it does var. 

 a. In it the head is represented as being bluntly rounded in front, 

 while in all the specimens in the lot upon which this account is based 

 there is a constriction near the anterior end which produces a blunt 

 button-like apex. The head in lateral view is therefore oblong and 

 not sagittate, as in Leuckart's figure. 



The terms marginal and lateral as applied to the head in this de- 

 scription designate those sides which correspond to the marginal and 

 lateral sides of the body, respectively, although this use of the terms 

 gives rise to the anomaly that the marginal diameter of the head is 

 greater than the lateral. 



A comparison of Fig. 6 with Fig. 9 might lead one to infer that there 

 is a great difference between the two varieties with respect to the ap- 

 pearance of segments at the posterior end. While this is true in many 

 H. Mis. 133 47 



