170 TREMATODA. 



the body, in the centre of which a median spine is placed. An x shaped 

 pigment spot, sometimes provided with a rudimentary lens (Monostomum 

 mutabile), is also generally situated on the dorsal surface. 



In some instances a more or less completely developed alimentary tract is 

 present (Monostomwin capitellum, Amphistomum subclavatum), but usually 

 there can only be distinguished in the interior of the larva a transparent 

 mass of cells bounded by a more or less distinctly marked body wall with 

 ciliated excretory channels. 



Ed. van Beneden has shewn that the ciliated covering is developed 

 while the embryo is still in the egg, and long before the yolk cells are com- 

 pletely absorbed. It would seem that even before hatching this ciliated 

 covering is to a great extent independent of the mass within. In the 

 larva of Monostomum mutabile (tig. 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 organised 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 au alimentary 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 commence- 

 ment 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 nu- 

 merous branches amongst the viscera of their hosts. 



The Rediae and Sporocysts rapidly grow in size and sometimes 

 increase by transverse division. In the course of their further de- 

 velopment one of two things may happen. They may either (1) de- 



