Ill GYMNOPLEA HETERARTHRANDRIA 59 



Calanidae in plumed ornaments, e.g. Attgaptilus filigerus, figured 

 by Giesbrecht in his monograph. The use of these ornaments, 

 which are possessed by so many pelagic Copepods, is entirely 

 obscure.^ Certain of the Centropagidae live in fresh water. Thus 

 Diaptomus is an exclusively fresh-water genus, and forms a most 

 important constituent of lake - plankton ; various species of 

 Heterocope occur in the great continental lakes, and certain 

 Euryteinora go up the estuaries of rivers into brackish water. 



An excellent work on the fresh- water Copepods of Germany 

 has been written by Schmeil,^ who gives analytical tables for 

 distinguishing various genera and species. The three fresh-water 

 families are the Centropagidae, Cyclopidae, and Harpacticidae 

 (see p. 62). The Centropagidae may be shai-ply distinguished 

 from the other fresh- water families by the following characters :- — 

 The cephalothorax is distinctly separated from the abdomen ; the 

 first antennae are long and composed of 24-25 segments, in the 

 male only a single antenna (generally the right) being geniculated 

 and used as a clasping organ. The fifth pair of limbs are not 

 rudimentary ; a heart is present, and only one egg-sac is found 

 in the female. The second antennae are distinctly biramous. 



Diaptomus. — The fiu'cal processes are short, at most three times as 

 long as broad ; endopodite of the first swimming appendage 

 2-jointed, endoj^odites of succeeding legs 3 -jointed. 



Heteroco'pe. — The furcal processes are short, at most twice as long as 

 broad ; endopodites of all swimming legs 1 -jointed. 



Eurytemora. — The furcal processes are long, at least three and a half 

 times as long as broad ; the endopodite of the first pair of legs 

 1-jointed, those of the other pairs 2-jointed. 



It has been known for a long time that some of the 

 marine Copepods are phosphorescent, and, indeed, owing to 

 their numbers in the plankton, contribute very largely to 

 bring about that liquid illumination which will always excite 

 the admiration of seafarers. In northern seas the chief 

 phosphorescent Copepods belong to Metridia, a genus of the 

 Centropagidae ; but in tlie Bay of Naples Giesbrecht ^ states 

 that the phosphorescent species are the following Centropagids : 

 Pleuromma ahdominale and P. gracile, Leuehartia Jlavicornis and 



^ They may assist the animal by retarding its sinking. Cf. Chun, " Aus den 

 Tiefen des Weltmeeres," 1905. 



^ Schmeil, BihliotJieca Zoologica, Hefte 11, 15, and 21. 

 3 Giesbrecht, Mitth. Zool. Stat. Neap, xi., 1895, p. 648. 



