12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



These terraqueous copepods form a new group intermediate between 

 the Others and differing from them in many ways. There is no neces- 

 sity for swimming in order to prevent them from sinking, and wher- 

 ever they may stop when moving about will provide a convenient 

 resting place. Their chief concern is to obtain an adequate food supply, 

 and in doing this they scurry about freely in the sand or mud. They 

 have much more freedom of motion than the commensal forms but 

 not nearly as much as the free-swimmers ; they may well be called 

 free-movers but scarcely free-swimmers. In all probability their time 

 is much more evenly divided between motion and rest than it is in 

 the other groups. 



Again, the free-swimmers, in consequence of their protracted move- 

 ments, cover a considerable area and may even be carried long dis- 

 tances by currents or drifts. In this way they are widely distributed, 

 and it is not uncommon to find some of the species in nearly every 

 ocean on the globe. The same thing is true of the parasitic copepods, 

 for here the females, and often both sexes, are carried about by the 

 hosts, and if the latter are fish or other animals capable of extended 

 migration, the parasites are thereby widely scattered. 



On the contrary, most commensal copepods live within ascidians, 

 holothurians. tunicates, and similar animals, which move about but 

 little if at all. And since the movements of the copepod adults are 

 also restricted, the species have only a limited distribution. Their 

 chance for dissemination lies in the escape of the larvae from their 

 host and the ability to swim about during their development stages. 



Similarly, in these terraqueous copepods the distance covered by the 

 locomotion of the adults is so limited that the distribution of the 

 species is seriously handicapped. The presence of a given species in 

 the sand or mud of one beach is no indication that it will be found in 

 neighboring beaches. We may go farther and say that the component 

 parts of the same beach are very likely to yield different species of 

 copepods. In short, isolation is as much an accompaniment of dwelling 

 in the sand or mud as is wide distribution a result of swimming freely 

 in the open ocean. 



A final consideration is concerned with reproduction and is also 

 intimately associated with distribution. Among the free-swimmers the 

 female carries her eggs about with her in external ovisacs or extrudes 

 them singly at intervals into the water. In the former case the eggs 

 are kept together until they hatch, in the latter case they are widely 

 scattered, since the female is constantly moving about while extruding 

 them. Similarly, when the eggs in the ovisac hatch, the nauplii do not 

 all emerge at the same time, but there is a considerable interval between 



