67 



relegate them to the species curtus as, other points being fairly 

 equally divided, the most outstanding feature, the relatively 

 large size of the male, marks them out as being nearer to C. curtus, 

 as it is the only species of C aligns in which this is the case. It 

 would appear, then, that we cannot draw a hard and fast line 

 between the tAvo species as we get such forms as these occupying, 

 an intermediate position, and, considering that the two species 

 occur on a number of hosts, amongst which Gadus morrhua is given 

 for each species, it would be only reasonable to suppose that the 

 two species — if they are actually separate species — are sufficiently 

 closely allied to breed one with the other, while it is possible that 

 a further investigation of the matter might show that according 

 to varjdng conditions of the host, and to differences in its sur- 

 roundings the parasites approach either the curtus form or the 

 rapax form, or that they may occupy an intermediate position 

 between these two extremes. 



The parasites infesting the carapace of this Caligus 23roved to 

 be the trematode Udonella j^oUuchii, Nob,f the largest of which 

 measured 4 mm. when iuWy extended. The}^ remained attached 

 to the edge of the carapace by the anal sucker, while alternately 

 contracting and extending the body, and twisting from side to 

 side very actively. The eggs of the trematode clustered beneath 

 the body of the Caligus are each club-shai^ed and suspended on 

 a short stalk. 



Masses of a Vorticella were also found attached to the carapace 

 of the Caligus, and in one case a small number had attached them- 

 selves to a trematode. 



0. M. JORGENSEN. 



EXPLANATION OF PLATE. 



Fig. 1. — Dorsal surface of female Caligus showing markings and egg -strings. 

 Fig. 2. — Same from ventral surface showing numbers of Udonella (Ud.) 



attached to edge of carapace and to egg-strings, also clusters of eggs 



of the trematode. 

 Fig. 3. — Two chromatophores. 

 Fig. 4. — Udonella pollachii. 

 Fig. 5. — Eggs of same. 



t Van Beneden, " Kecherches sar les Bdellodes on HiruJinLjes ct las Trem\toae,s ilaiui;:-.' 



