592 THE BIOLOGY OF MARINE ANIMALS 



live on or within a host and derive their nutriment from the latter's tissues 

 or fluids. As distinct from predators they do not immediately kill the animal 

 upon which they subsist, although they may ultimately encompass its 

 destruction by severely reducing its tissues or drastically curtailing its 

 nourishment. There are, of course, many border-line cases which are 

 referred by usage to commensalism, parasitism or symbiosis, and many 

 which require further study for clarification. 



In some groups — for example sporozoans and cestodes — all members 

 are parasitic, whereas others contain only a few species which have 

 assumed a parasitic existence. Among the latter are many excellent ex- 

 amples illustrating graduated modifications towards parasitic specializa- 

 tion. The numbers of parasites are incredibly vast: indeed it has been 

 estimated that since every animal contains several species of parasites, the 

 latter, in species and individuals, must far outnumber the free-living 

 animals (44). Of that multitude several have been chosen to provide an 

 introduction to parasitism among marine animals. 



Coelenterates and Ctenophores. Some interesting parasitic species occur 

 among coelenterates. Ectoparasitic hydroids which live on the skin of 

 fishes have already been mentioned (p. 583). A minute anthomedusan 

 Mnestra, bearing abortive tentacles, is found attached to the nudibranch 

 Phyllirrhoe (Fig. 13.6). This relationship, originally considered to be one 

 of parasitization of the snail by the medusa, now receives a reverse explana- 

 tion. Mnestra is regarded as the degenerate medusa of Zanclea and other 

 coastal hydroids, which Phyllirrhoe utilizes as a pelagic vehicle in early 

 stages of its life-history. As Phyllirrhoe matures it dispenses with its starved 

 and degenerate host. Some of the Narcomedusae have a life-history 

 complicated by parasitism in the actinula stage. The planulae, as in 

 Cunina, attach themselves to other Trachymedusae or Hydromedusae 

 where they live parasitically and bud off medusae. And larval actinians of 

 the genus Peachia live for a time attached to the tissues of various Hydro- 

 medusae and Schyphomedusae (Cyanea, Aequorea, Catostylus) before 

 taking up a benthic existence (2, 8, 61, 79, 93). 



Among ctenophores there is a form, Gastrodes, which passes through a 

 parasitic stage in the mantle of Salpa. After a while it forsakes the salp, 

 settles on the bottom and metamorphoses into adult form. Infection of a 

 new host takes place by a planula larva (60). 



Platyhelminthes and Nemathelminthes. Platyhelminth parasites infest a 

 wide variety of marine animals, and include forms which are confined to a 

 single host, and others which pass through a complicated life-cycle involv- 

 ing two or more successive hosts. These include an intermediate host in 

 which the primary stages of the parasite develop, and a definitive host in 

 which the parasite becomes sexually mature. The classical examples of 

 platyhelminth parasitology derive largely from studies on terrestrial and 

 freshwater animals. Those trematodes and cestodes found in strictly 

 marine forms yield no new principles that are not already embraced in 

 parasitological treatises, and only a few marine examples will be mentioned. 



