200 FLEAS, FLUKES AND CUCKOOS 



and will only attempt entry into the "right" host. Some molluscs, for 

 instance the scallops [Pecten)^ never harbour larval flukes, and appear to 

 be entirely immune to their attacks. Once inside the tissues of the snail- 

 host the miracidium degenerates into a hollow sac and a complicated 

 type of asexual multiplication follows, the exact nature of which is still 

 not understood. One theory supposes that the germ cells, which are car- 

 ried within the body of the miracidium, segment and subsequently 

 fragment; and these fragments give rise to the different types and differ- 

 ent generations of larvae which develop in the snail (germinal lineage 

 with polyembryony) . There are numerous other theories, none of which 

 is satisfactory. The larval form following the miracidium is an immobile 

 simple sac-like structure known as a mother sporocyst. The next 

 generation in the case of many species, including the redshank fluke, 

 are larvae of a more complicated type which are called rediae. These 

 are hollow worm-like forms, which possess a pharynx, primitive gut, 

 specialised secretory cells, excretory system, and an ambulatory process. 

 They are capable of a limited amount of movement, and feed actively 

 upon the tissues of the host. Within their body cavity daughter rediae 

 are developed, resembling the mother rediae, which emerge through a 

 birth pore and add to the population, feeding and growing in the 

 reproductive and digestive organs of the snail. Several of such genera- 

 tions are produced and then, for some reason not properly understood, 

 the germ cells dispersed in the bodies of the various rediae, give rise to a 

 different type of larva known as a cercaria. When these cercariae reach 

 a certain stage of development they emerge from the rediae and con- 

 tinue development in the tissues of the snail. Eventually they work 

 their way along certain well defined routes, such as the circulatory 

 system, and escape into the water. 



The classical type of cercaria is similar in shape to the adult fluke 

 but provided with a tail. It does not remotely resemble the redia in 

 which it developed. Certain fundamental anatomical features — the 

 excretory system, suckers and gut — characteristic of the adult worm 

 can generally be observed in the cercaria. Many adaptations connected 

 with the free-swimming phase and the entry into the second inter- 

 mediate host, such as the tail, fins, eye-spots, penetration glands, boring 

 spines and cystogenous glands, are generally present. 



In the case of the redshank fluke, thousands of cercariae emerge 

 from one spire shell — all the progeny of a single ^gg. They are just 

 visible to the naked eye, and, hanging motionless in the water, they 



