594 



THE BIOLOGY OF MARINE ANIMALS 



of their life-cycles, and certain forms also make use of successive hosts. In 

 the ascaroid Contracaecum, for example, the eggs begin their development 

 in fish or invertebrates such as Sagitta, pass through other fish acting as 

 transport hosts, and finally reach maturity when ingested by predaceous 

 fish or fish-eating birds and mammals (50). 



Fig. 14.8. Life-cycle of the Fluke Parorchis acanthus 

 (a) Adult fluke from herring gull; (b) miracidium; (c) first-generation redia; (>f) second- 

 generation redia from the dogwhelk Nucella; (e) echinostome cercaria, which penetrates 

 a mussel or cockle; (/) cyst from one of the latter bivalves, ((a) After Rees (1939); 

 (b), (c) and (d) after Rees (1940); (<?) and (/) after Lebour and Elmhirst (1922), and 

 Dawes (19).) 



Mesozoa. These organisms comprise the dicyemids and orthonectids, 

 both of which are simple internal parasites of invertebrates. Their compli- 

 cated life-cycles involve an alternation of sexual and asexual generations, 

 and the production of agametes. 



Dicyemids are parasites found in the kidneys of cephalopods. They 

 infect a cephalopod shortly after it hatches from the egg, and in the young 



