METRIDIA. 41 



segments coalescent. Abdomen in the female consist- 

 ed 



ing of three, in the male of five segments. Anterior 

 antennas as long as the cephalo thorax, 2 5 -jointed in 

 the female, in the male 20-jointed and furnished with 

 a hinge between the seventeenth and eighteenth joints. 

 Second pair of antennas as in Oalanus ; mouth organs 

 as in Galanus. First pair of feet smaller than the 

 rest : both branches of the first four pairs of feet 

 three-jointed; fifth pair, unlike the others, composed 

 of one branch only and dissimilar on the two sides ; in 

 the male formed for grasping. 



The characters on which Boeck relies to separate 

 this genus from Pleuromma, Claus, are the complete 

 separation of the cephalic from the thoracic segments, 

 the 3-jointed branches of the swimming feet, and the 

 absence of lateral eye-spot. It is possible that further 

 investigation may show these distinctions to be un- 

 founded, but the male abdomen as figured by Claus in 

 the case of Pleuromma is distinctly different from that 

 of Metridia in being provided with lateral processes. 

 M. Boeck, however, is wrong in stating the number 

 of antennal joints in Metridia to be twenty-four. The 

 real number is twenty-five, and in this respect it 

 agrees with Pleuromma. 



While this Monograph has been going through the 

 press Mr. Norman has pointed out to me that the 

 name Metridia (Metridium being already in use for a 

 genus of Actinozoa) must be withdrawn, but as it 

 seems doubtful whether Pleuromma ought or ought 

 not to be adopted as the generic appellation, I prefer 

 for the present to let Metridia stand. 



