NO. 10 



CRUSTACEAN METAMORPHOSES — SNODGRASS 



21 



(H) known as the head, or cephalosome, and a posterior thoracic 

 region of five segments. The head carries the two pairs of antennae, 

 the mandibles, two pairs of maxillae, and the first pair of legs, or 

 maxillipeds. The five segments of the thoracic region bear each a pair 

 of legs, but the legs of the last pair may be much reduced. The 

 genital ducts open on the basal segment of the abdomen. 



The free-swimming copepods occur in such vast numbers in the 

 ocean and in some inland lakes that they constitute a most important 

 food source for many other aquatic animals from arrowworms to 

 whales, but particularly for fishes. Being minute creatures themselves, 

 the free copepods feed on the microscopic plant life of the water, 



2Mx Mxpd' 



Fig. 6. — Copepoda. Calanus cristatus Kroyer, adult. 

 I Ant, first antenna; 2Ant, second antenna; H, "head"; Md, mandible; iMx, 

 first maxilla; 2Mx, second maxilla; Mxpd, maxilliped; VI, VII, XI, body 

 segments. 



which, elaborated in their own bodies, is thus passed on as food for 

 the larger animals. It w'ould seem, however, that the copepods have 

 retaliated on the animals that eat them, since many species have be- 

 come parasites of their potential enemies. Though fish are their favor- 

 ite hosts, the parasitic copepods are not discriminative and attack 

 almost every kind of creature that lives in the ocean. On the other 

 hand, the copepods themselves are infested by numerous parasites, 

 even by some of their own kind. Evidently life in the ocean is not 

 a happy existence for either the predators or their victims. 



The nonparasitic copepods go through no changes of form in their 

 life histories that can truly be called a metamorphosis. Their environ- 

 ment is practically the same at all periods of their lives, and there 

 is no call for adaptive modifications in either the larval or the adult 

 stage. The successive developmental stages are merely steps in growth 

 from youth to maturity. As an example, we may take the fresh-water 



