III GYMNOPLEA—_-HETERARTHRANDRIA 59 
Calanidae in plumed ornaments, e.g. Augaptilus 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 
Hurytemora 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 sharply distinguished 
from the other fresh-water familes 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 lmbs 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 fureal processes are short, at most three times as 
long as broad; endopodite of the first swimming appendage 
2-jointed, endopodites of succeeding legs 3-jointed. 
Heterocope-——The furcal processes are short, at most twice as long as 
broad ; endopodites of all swimming legs 1-jointed. 
Eurytemora.—The fureal 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 
ao : sate ; 
phosphorescent Copepods belong to Metridia, a genus of the 
Centropagidae ; but in the Bay of Naples Giesbrecht ° states 
fo) >] 
that the phosphorescent species are the following Centropagids : 
Pleuromma abdominale and P. gracile, Leuckartia flavicornis and 
-1 They may assist the animal by retarding its sinking. Cf. Chun, ‘‘ Aus den 
Tiefen des Weltmeeres,”’ 1905. 
2 Schmeil, Bibliotheca Zoologica, Hefte 11, 15, and 21. 
3 Giesbrecht, Mitth. Zool. Stat. Neap. xi., 1895, p. 648. 
