46 THE VOYAGE OF H.M.S. CHALLENGER. 



fifteenth and sixteenth joints; posterior antennae and mouth-organs as in Calanus. 

 First pair of feet much smaller than the rest; both branches of the first four pairs three- 

 jointed; fifth pair composed of one branch only, prehensile in the male. A black, 

 spherical eye spot, covered with a highly refracting lens, is situated on the side of the 

 body, near the base of one of the foot-jaws. 



Amongst other characters of his genus Pleuromma, Dr. Claus states that the head 

 and thorax are coalescent, and that the inner branch of the first pair of swimming feet is 

 composed of only two joints. Boeck, therefore, relying upon the distinctly separated head 

 and thorax, upon the three-jointed inner branch of the first foot, and upon the absence of 

 a pleural eye, necessarily assigned his new species Metridia armata to a distinct genus. 

 But, while hesitating to disagree with so accurate an observer as Dr. Claus, I am bound 

 to say that I believe his definition of Pleuromma to be founded — so far as regards the 

 fusion of the head and thorax, and the jointing of the first pair of feet — on mistaken 

 observation, or, it may be, on data derived from immature specimens. On the other hand, 

 specimens of Pleuromma may frequently be found in which no lateral eye is visible (at 

 any rate in spirit specimens), while the inconstant position of this organ, sometimes on the 

 right side and sometimes on the left, seems to sanction the supposition that it is to some 

 extent an extraneous appendage. I therefore think, taking one consideration with 

 another, that there is no sufficient reason to maintain the separation of the two genera, 

 and though doubtless Pleuromma is a misnomer when applied to species having no 

 pleural eye l , it must be adopted on the ground of priority. 



Pleuromma abdominale, Claus (PI. XL figs. 1-13, and PI. XII. figs. 1-16, and 



PI. XXXI. figs. 13, 14). 



Pleuromma abdominale, Claus, Die frei lebenden Copepoden, p. 195, pi. v. figs. 1-6, 13, 14, 



pL vi. figs. 1-10. 

 Diaptomus aldominalis, Lubbock, Trans. Entom. Soc, voL iv. (1856), p. 22, pi. x. figs. 1-8. 

 (I) Pleuromma gracile, Claus, Die frei lebenden Copepoden, p. 197, pi. v. figs. 7-11. 



Length, l-8th of an inch (3 mm.). Cephalothorax elongated, moderately robust; 

 anterior antennas twenty-five-jointed, about as long as the body, towards the base densely 

 clothed with rather short setas, interspersed with others of moderate length; the second 

 and third joints in the female each armed with a sharp, recurved marginal spine, the 

 following six or eight joints doubly denticulated (PI. XL fig. 2). The right anterior 

 antenna of the nude (fig. 4) has a strongly denticulated plate (fig. 5) on the proximal 

 side of the hinge-joint, the sixth joint is imperfectly divided into three, and the tenth, 

 eleventh, and fourteenth, are angularly dilated at the apices. The inner branch of the second 

 pah' of feet, on the left side only in the male (fig. 6), but on both sides in the female, has 



1 In the northern species, Metridia (Pleuromma) armata, there is no pleural eye. 



