56 CRUSTACEA——COPEPODA 
the mouth and its appendages, dividing the free-living and 
semi-parasitic forms as Gnathostomata from the true parasites or 
aca 
& AY eal 4 
Sy, wun HY 
Fic. 25.—Calanus hyperboreus, x 30. Abd}, First abdominal segment; Ist Ant, 
2nd Ant, Ist and 2nd antennae ; Md, mandible ; Ma, Mz?, 1st and 2nd maxillae ; 
Map, maxillipede ; Th1, 1st thoracic appendage. (After Giesbrecht.) 
Siphonostomata. This division, although convenient, breaks 
down in many places, and it is clear that the parasitic mode of 
life has been acquired more than once in the history of Copepod 
