of the Fishery Board for Scotland. 



147 



The first three pairs of thoracic feet have both branches apparently 

 three-jointed, but the outer branches are rather shorter than the inner 

 ones (fig. 5 represents the first pair). In the fourth pair, the first two 

 joints of the outer branches appear to be coalescent, so that they seem to 

 be only two-jointed, but the inner branches, as in the preceding three 

 pans, are three-jointed (fig. 6). The abdomen is short but moderately 

 stout; in the adult female it seems to be shorter than it really is, from 

 being partly hidden by the enlarged cephalothorax ; the first abdominal 

 segment is rather broader than the next and is longer than the entire 

 length of all the others ; the caudal segments are short and the principal 

 furcal setae are stouter and much more elongated than those adjacent. 



Length about '8mm. (^ of an inch). 



Habitat. — Found adhering to the inner surface of the gill-covers of a 

 three-spined stickleback (Gasterosteus aculeatus), captured in Sinclair 

 Loch, Barra, Outer Hebrides, in May 1894 ; and on another taken in the 

 Kiver Forth, near Alloa, February 1896. The same species of Copepod 

 was also found on the inside of the gill-covers of a fifteen-spined stickle- 

 back (Gasterosteus spinachia), captured in Loch Etive in May 1896. 



Remarks. — This Copepod has by some writers been ascribed to the 

 genus Ergasilus, but it does not fit in very readily with the characters of 

 that genus. The typical Ergasilus has the body moderately elongated, 

 whereas in the present form the body is, except for the short abdomen, 

 not only short but almost spherical in general appearance. I therefore 

 agree with Dr. Canu's restoration of Pagenstecher's generic name, 

 Thersites, for this species. 



Fam. Caligid/E. 



The Caligidse contain a much larger number of genera and species 

 than any other of the seven families amongst which the Copepod 

 parasites of fishes have been arranged. According to Dr. Basset-Smith, 

 the number of genera belonging to the Caligidse is (exclusive of Nogagus) 

 25, while the number of species is 124. Two of the genera — C aligns and 

 Lepeophtheirus — contain 49 and 26 species respectively, or 75 in all, which 

 is fully three-fifths of the total number belonging to the whole family. As 

 for Nogagus, no female Copepod which could satisfactorily be ascribed to 

 this genus has ever been observed, and students of the Copepoda are now 

 of opinion that the various "species" of Nogagus are really the males of 

 species belonging to other genera, i.e. : Pandarus, Dinematura, etc. 



Steenstrup and Liitken, in their valuable memoir on Copepod parasites 

 of fishes,* include under Caligus the species belonging to that genus and 

 also those belonging to Lepeophtheirus. The various forms belonging to 

 these two groups have a close resemblance to each other, not only in their 

 general appearance, but also in their structure ; but there are two characters 

 by which the species belonging to the one group are easily separated from 

 those belonging to the other ; these characters are the presence or absence 

 of sucking disks or lunuhe on the frontal plates, and the unifid, or bifid 

 form of the appendages described by Steenstrup and Liitken as palpi. 

 In Caligus, lunulce are always present and the palpi are unifid, in 

 Lepeophtheirus, on the other hand, Iwiulce are always absent and the 

 palpi are bifid. The importance to be attached to these differences is, of 

 course, very much a matter of opinion, and Lepeophtheirus will by authors 

 take its place as a "genus" or be reduced to a synonym of Caligus, 

 according to the value they place on these differences. In the present 

 paper I follow Dr. Baird and Dr. Basset-Smith in regarding the two as 

 distinct genera. 



* Sserskilt aftrykt Kongl. Danske Vidensk-Selsk. Skrifter. 5te Rfekke Naturh. o. 

 Matheinat. Afdel. 5te Bind. 



