148 



Tart III. — Eighteenth Annual Report 



Genus Caligus, 0. F. Miiller (1785). 



(1) Abdomen in the female composed of one segment. 



The general outline of the body is more or less oval and greatly- 

 depressed. Lunula, or " sucking disks," are present on the frontal plates ; 

 the palpi are thorn-like, and end in a single point. Fourth pair of 

 thoracic feet one-branched. 



Caligus curtus, Miiller. (PI. V., figs. 8-12.) 



1785. Caligus curtus, Miiller, Entomostraca, p. 130, PI. 

 XXI., fig. 1. 



1850. Caligus miilleri (?) and diaphanus, Baird, Brit. Entom., 

 pp. 271 and 269.* 



The Caligus miilleri and probably also Caligus diaphanus of Dr. 

 Baird's " Entomostraca " are, according to Steenstrup and Liitken, 

 synonymous with Caligus curtus (Miiller), and this seems to be also the 

 opinion of recent writers on this group of Copepods. In view of this, 

 however, it may be noted that in most, if not in all, the other species of 

 Caligus and Lepeophtheirus the male is usually, and sometimes consider- 

 ably, smaller than the female ; but the Caligus diaphanus of Baird, 

 which is considered to be the male of Caligus curtus (Miiller), is frequently 

 very much larger than the ovigerous females of that species. Some 

 species are, however, subject to more variation than others, and this may 

 account, partly at least, for the difference in size. 



Caligus curtus is not only a common species, but may be found on 

 various kinds of fishes, including several species of Gadus, ling, hake, 

 torsk, plaice, and other flat-fishes ; the grey and the long-nosed skate, etc. 



A certain amount of variableness in size and general appearance is 

 observable amongst the different specimens that have beeu examined ; 

 but the difference was not greatly in excess of that which has been noticed 

 in specimens belonging to some of the other species. 



Caligus rapax, M.-Edw. (PI. V., figs. 13-19.) 



1840. Caliqus rapax, M.-Edw., Hist. Nat. Crust., vol. iii., p. 453, 



PI. XXXVIII., fig. 9. 

 1850. Caligus rapax, Baird, op. cit., p. 270, PI. XXXII., ligs. 



2 and 3. 



This, like Caligus curtus, is one of the more common species, but it 

 differs from most other Caligi, at least to some extent, in being much 

 more commonly obtained as a free-swimmer in the open sea. We 

 frequently find Caligus rapax in tow-net gatherings, collected both at the 

 surface and at the bottom. Ovigerous females are, however, not so 

 commonly met with as males and young females. But though it is 

 apparently more of a free-swimmer in its habits than others, and also 

 perhaps because of this, Caligus rapax is found on even a larger variety 

 of fishes than Caligus curtus. There is scarcely a fish in our seas on which 

 this Caligus may not at one time or another be found. • 



Like Caligus curtus, this species also exhibits a certain amount of 

 variation in the forms and sizes of males and ovigerous females, but it is, 

 if anything, more marked in the latter. Some females with ova from a 

 large grey skate varied from 54mm. to 6*lmm. in total length ; while the 

 length of the abdomen in the same specimens varied from 0'7mm. to 1*1 mm. 

 Some difference was also observed in these specimens in the breadth of 

 the last segment of the thorax. Two apparently adult males obtained on 



*For other synonyms, see Basset-Smith's "Systematic Description" already referred to. 



