INVERTEBRATA. 359 



the resistant shell protects them from the action of the 

 digestive secretions. Passing along the course of the in- 

 testine they are finally ejected from the body of the sheep 

 with the faeces. The embryo within the egg-shell then 

 begins to develop if the necessary conditions are present. 

 These conditions are moisture and an ordinary summer 

 temperature. Thus the multiplication of the parasite goes 

 on in damp pastures in the summer season, but is checked 

 in winter, in dry weather, or on dry ground. The eggs may 

 develop in wet meadows in rainy weather, or in ditches and 

 pools. The egg-shell contains originally a small proto- 

 plasmic ovum and a large number of spherical yolk 

 cells, which serve as food for the developing embryo. 

 One end of the egg-shell forms an operculum, which 

 becomes detached when the larva is fully developed 

 (Fig. 187). 



6. Larva. The newly hatched larva of the liver-fluke 

 is of conical shape and uniformly ciliated all over its sur- 

 face. By the active movements of the cilia the larva swims 

 about in the water. The broad end is the anterior, and in 

 the centre of this broad end is a small papilla, which can 

 be extended till it is long and pointed, or contracted so that 

 it is short and blunt. The most external layer of the body 

 consists of large flat cells which bear the cilia. These cells 

 are hexagonal in shape and are disposed in five transverse 

 bands. Beneath the ciliated cells is a layer of nucleated 

 protoplasm in which cell outlines are difficult to distinguish. 

 In this layer are circular and longitudinal muscle-fibres. 

 Anteriorly on what may be called the dorsal side are a 

 pair of eye-spots, each consisting of a pigmented cell pro- 

 vided with a lens-like particle of refractive substance. 

 These eye-spots are beneath the external ciliated cells. 

 There are also a pair of rudimentary nephridia, each con- 

 sisting of a single flame-cell. The central part of the body 

 consists of large rounded cells, which in later stages of the 

 Life-history develop into new individuals, and are therefore 

 reproductive cells or germ-cells. The external and internal 

 structure of the larva are shown in the second and third of 

 the diagrams in Fig. 187. 



