24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I31 



fifth stage, with traces of the fifth and sixth pairs of appendages. In 

 the sixth instar, judging from related species, there are present second 

 maxillae, maxillipeds, and two pairs of swimming legs. The next 

 instar is that of the first copepodid, which has three pairs of legs ; the 

 fourth legs appear in the second copepodid instar, and the definitive 

 number of five is present in the third copepodid. The fifth and last 

 copepodid is essentially like the adult. The free-swimming copepods, 

 therefore, have a typical anamorphic development. Being crustaceans, 

 they are primarily constructed for life in the water, and so long as 

 they maintain a free existence there is no need of metamorphic 

 adaptations to any other way of living. 



When now we turn to the parasitic copepods, the story is very 

 different. An aquatic animal that hatches as a freely swimming larva 

 and then becomes sedentary on another animal from which it extracts 

 its food changes its environment and its mode of living in a very 

 radical way. In some manner difficult to understand metamorphic 

 changes of structure have been evolved that adapt the parasitic animal 

 to its life of parasitism, and in many cases the transformation has 

 been carried so far that the adult parasite could not be identified, or 

 even recognized as a crustacean, if its early stages were not known. 



A few copepods appear to be transitional in their habits between a 

 free life and one of parasitism. Such species are termed semiparasitic 

 by Wilson (1921b), who says they are found on worms, mollusks, 

 echinoderms, and in the gill chambers of crabs. These species are 

 capable of swimming freely in the water, and their residence on any 

 one host may be temporary. Their mouth parts, according to Wilson, 

 are not suitable for either chewing or sucking and appear to be adapted 

 for licking nourishment from the animals to which they attach them- 

 selves. A species with biting mouth parts, however, could hardly 

 resist sampling the blood of its host and then becoming an habitual 

 parasite. 



The truly parasitic copepods include a large number of species, all 

 of which undergo striking metamorphic adaptations to the nature 

 of the host or the part of the host attacked, and some of them lead 

 a double life on two diflferent species of hosts. Some parasitic copepods 

 undergo their metamorphoses during the larval development and 

 become again free living in the adult stage ; others remain on the host 

 and attain their highest degree of metamorphosis as adults. Most of 

 them, however, hatch from the eggs as typical nauplii, and in this 

 stage or the following copepodid stage they must find their proper 

 hosts. 



