ASSOCIATIONS 593 



Trematodes. Monogenetic trematodes are ectoparasites that develop on 

 a single host. They include such forms as Isancistrum, a parasite of Loligo; 

 the hyperparasite Udonella, found on Caligus, itself a copepod parasite of 

 various marine teleosts; and Trochopus, occurring on gurnards and sea- 

 bream (63a, 99). The Aspidogastrea are found as endoparasites in turtles, 

 fish and certain invertebrates. 



Digenetic trematodes are endoparasites which require two or more 

 hosts to complete their life-cycle. The larvae (miracidia) on hatching infect 

 a temporary or intermediate molluscan host, where they pass through 

 stages of sporocyst (sometimes omitted), redia and cercaria. Frequently, 

 the cercariae forsake the host mollusc on completing their growth and 

 encyst, and it is the cyst which infects the final host. In some species the 

 cercariae, instead of encysting externally, form cysts in the tissues of the 

 same or some other kind of intermediate host, where they are known as 

 metacercariae. The second intermediate host is often another species of 

 mollusc, but some trematodes also utilize Crustacea and fish, and meta- 

 cercaria larvae have also been found in such invertebrate hosts as poly- 

 chaetes, echinoids and holothurians. 



The fluke Parorchis acanthus, which has been studied in some detail, 

 will illustrate some of the intricacies of trematode life-histories (Fig. 14.8). 

 The adult Parorchis is a parasite occurring in the rectum and bursa Fabricii 

 of the herring gull. The eggs hatch shortly after they are laid and yield 

 active ciliated miracidia which attack and penetrate the dog-whelk Nucella 

 lapillus. In the whelk the miracidia give rise to two generations of rediae, 

 which feed upon the tissues of the mollusc. Eventually these form echino- 

 stome cercariae which forsake the whelk and penetrate a second inter- 

 mediate host, either Mytilus or Cardium, and form cysts in the foot and 

 mantle. Finally the gulls become infected by eating parasitized mussels and 

 cockles (19). 



Trematodes produce manifold superficial and profound changes in the 

 intermediate and definitive hosts. Heavy infestations with larval trematodes 

 lead to extensive destruction of gonads or liver in gastropods, and these 

 animals may also respond to heavy infections by considerable increases in 

 size. Some animals may develop immunity to trematodes as the result of 

 repeated attacks, and resistance sometimes in&eases with age (19, 20, 

 49, 70). 



Cestodes. In the adult stage cestodes are little more than reproductive 

 sacs which absorb nutriment through the general body surface. They are 

 almost exclusively parasites of vertebrates and occur in the alimentary 

 canal, although the larvae are widespread in all regions of invertebrate and 

 vertebrate hosts. In many cestodes two hosts are required to complete the 

 life-cycle, of which the first is a crustacean, the second frequently a species 

 of fish. Larval stages of the diphyllidian Echinobothrium, for example, are 

 found in various marine molluscs and crustaceans, whereas adults inhabit 

 the intestine of elasmobranch fishes (49, 51, 98). 



Nematodes. Parasitic nematodes rival the platyhelminths in the character 



