206 TREMATODA. 



larva of Monostomum mutabile (fig. 95 A), which offers an example of 

 an extreme case of the kind, there is present within the ciliated epidermis 

 a fully-developed independent worm. 



The non-ciliated larvae are less highly organized than the ciliated forms, 

 and are covered by a cuticle : their anterior extremity is sometimes provided 

 with a circular plate armed with radiate ridges and spines. 



The free-swimming or creeping embryos make their way into 

 or on to the body of some invertebrate (occasionally vertebrate) 

 form, usually a Mollusc, to undergo the first stage in their 

 metamorphosis. They may either do this on the gills of their 

 host, or very frequently they bore their way into the interior of 

 the body. Soon after the larvae have reached a satisfactory 

 position the epidermis becomes stripped off, and there emerges a 

 second larval form developed in the interior of the first larva, 

 much as a Nemertine is developed within the larva of Desor. 

 In the case of Monostomum mutabile the new worm is, as 

 stated above, fully formed within the ciliated larva at the time 

 of hatching. 



The worm which proceeds from the above metamorphosis 

 has different characters corresponding with those of the larva 

 from which it proceeded. If the original larva had an alimen- 

 tary canal it has one also, and then grows into the form known 

 as a Redia (Fig. 95, B and C). 



The Redia has anteriorly a mouth leading into a muscular 

 pharynx and thence into a caecal stomach. Posteriorly the body 

 is prolonged into a kind of blunt caudal process, at the com- 

 mencement of which are a pair of lateral papillae. There is a 

 perivisceral cavity, and the body walls are traversed by excretory 

 tubes. 



If the original larva is without an alimentary tract, the 

 second form becomes what is known as a Sporocyst. The 

 Sporocyst is a simple elongated sack with a central body cavity ; 

 when derived from the metamorphosis of a ciliated embryo its 

 walls are provided with excretory tubes, but such tubes are 

 absent in Sporocysts developed from non-ciliated larvae. Some 

 Sporocysts send out numerous branches amongst the viscera of 

 their hosts. 



The Redias and Sporocysts rapidly grow in size and some- 

 times increase by transverse division. In the course of their 



