396 



INVERTEBRATE MORPHOLOGY. 



1. Suborder Eucopepoda. 



This suborder includes the majority of the Copepoda, and 

 its members are characterized by having only the first thoracic 

 segment fused with the head and by possessing usually a 

 well-developed abclomeu. Many are free-swimming, some in- 

 habiting fresh water, as Cyclops and Canthocamptus, while 

 others are more especially marine, such as Harpacticus, Calanus 



B 



FIG. 178. A, Philichfhys xiphice SEEN FROM THE DORSAL SURFACE (after 

 CLAUS); B, Achtheres percarum (from BRONN). 



(Fig. 177), and Cetochilus, the latter sometimes occurring in 

 enormous schools, and forming an important food-supply for 

 fish and the baleen whales. Some, on the other hand, lead a 

 commensalistic life, occurring in the branchial chamber of 

 Tuuicates, e.g. Notodelphys, while a large number of forms are 

 parasitic. The degree of parasitism varies greatly in different 

 forms ; thus many are capable of free existence, becoming 

 parasitic only occasionally, such asG'on/coews and the brilliantly- 

 colored Sapphirina, while others, such as Ergasilus, parasitic 

 on the gills of fishes, and Caligus and Pandarus, though essen- 



