30 Guide to Crustacea. 



Table-case important part in the economy of nature than any otlier 



No. 2. Crustacea. 



. Many Copepoda hve as parasites on fishes and other aquatic 

 animals, and as a result of this parasitic life their structure 

 becomes greatly modified and degenerate. 



The Order Eucopedoda (Fig. 11) includes the great majority 

 of the Copepoda, both free-living and parasitic. True paired com- 

 pound eyes are never present, but the median unpaired eye is 

 often well-developed. Most of the free-swimming species are 

 extremely minute, few^ attaining the size of Eiichacta norvegica, of 

 which specimens are exhibited. Tlie enlarged drawings show the 

 brilliant colours of some pelagic species. 



Calocalanus pavo, one of the free- swimming Copepoda of the " plankton." 

 Enlarged. (From Lankester's "Treatise on Zoology," after Giesbrecht.) 



The parasitic species are usually much larger than those 

 which live a free life, and a number of species taken from common 

 fishes are exhibited. Pennclla, wdaich is found on whales and 

 fishes, is the giant of the sub-class, some specimens being even 

 larger than that exhibited here. 



The order Branchiura includes a small number of fish- 

 parasites whose exact relations to the other Copepods are obscure. 

 They possess a pair of compound eyes, and a piercing stylet, con- 

 nected with a poison-gland, in front of the mouth. ArgvJua 

 foliaccvs is common on fresh-water fishes in this country. The 

 large Argnlim scutiformis is taken from marine fishes in Japan. 



