of the Fishery Board for Scotland. 



149 



the same fish with the females show a difference of at least 0*7 mm. in 

 total length. These differences are more clearly exhibited in tabular form 

 as follows : — 



Variation in Size of Caligus rapax from Rata batis. 





Female Specimens. 



Male Specimens. 



Total length from fore- 

 head to the end of 

 the caudal furca, 



No. 1. 

 5 '2mm. 



No. 2. No. 3. 

 6'lmm. 6'2mm. 



No. 4. 

 6'lmm. 



No. 1. No. 2. 

 3'3mm. 4'5mm. 



Length of abdomen, 



•7mm. 



•8mm. '8mm. 



l'lmm. 





Breadth of last thoracic 

 segment, . 



'8mm. 



'8mm. l'Omm. 



1 '1mm. 





Though the length of these female specimens varied to the extent of a 

 full millimetre, there appeared to be no structural difference of any 

 importance. Perhaps it may be noted in passing that fig. 2, PL 

 XXXII., in Dr. Baird's work represents a young female and not a 

 male — an error due no doubt to an oversight on the part of the artist. 



(2) Abdomen composed of two segments. 



Caligus diaphanus, Nordmann. (PL V., figs. 20-25.) 



1832. Caligus diaphanus, Nord., Mikrog. Beitrage, ii., p. 26 



(non C. diaphanus, Baird). 

 1894. Caligus isonyx, T. Scott, Twelfth Ann. Rept. Fish. Board 



for Scot. (III.), p. 310. 

 1896. Caligus diaphanus, Basset-Smith, Journ. M. B. Assoc., 

 Plymouth, p. 156. 



Both male and female of this species have the abdomen two-jointed. 

 The species is a small one; the female specimen represented by the 

 drawing (fig. 20) is only about 4'4mm. in length ; the abdomen is 

 moderately elongate, being about one-fourth of the entire length of the 

 animal ; the last abdominal segment is small but quite distinct. The 

 lateral margins of the last thoracic segment are convexly and evenly 

 rounded, but posteriorly this segment is abruptly truncate, and is also 

 not much longer than the abdomen. 



The antennae (posterior antennae) are moderately short and armed 

 each with a strongly but evenly hooked claw. The mandibles are 

 slender and elongated, and the last joint is distinctly serrated on the 

 inner edge (fig. 22). The posterior foot-jaws are robust, and furnished 

 with strong terminal claws ; the penultimate joint is also produced near 

 the base of the inner margin into a stout triangular tooth (fig. 24). The 

 sternal fork has moderately long and slightly divergent branches (fig. 23). 

 The apices of the palpi are single and spine-like. 



The fourth pair of thoracic feet are moderately stout ; the branches are 

 distinctly three-jointed, and carry five moderately long setae (fig. 25). 

 There is also on the last thoracic segment what appears to be a fifth pair 

 of feet, each of which consists of a minute sub-triangular one-jointed 

 appendage bearing a few small setae. The caudal segments are very short. 



In the male the abdomen consists of two sub-equal segments ; the last 

 thoracic segment is narrow and not much wider than the abdomen. The 

 antennae appear to differ slightly from those of the female, especially in 

 possessing a supplementary tooth near the base of the terminal claw. 



