INVERTEBRATA. 361 



while its body is rotated by the motion of its cilia. Inside 

 the tissues of the snail the larva throws off its external 

 covering of ciliated cells and develops a large cavity in its 

 interior, lined by the germ cells. The anterior papilla dis- 

 appears and the eye-spots degenerate, so that the larva is 

 converted into a simple sac of somewhat indefinite elon- 

 gated shape. In this stage the animal is called a sporocyst, 

 which grows to a length of about 5 mm., and occasionally 

 may be divided to form two sporocysts. 



8. The Redia. Within the cavity of the sporocyst 

 the germ-cells become free and undergo development like 

 ova, but without fertilisation. We have, therefore, here 

 a case of parthenogenesis (parthenos, virgin; genesis, 

 generation). The germ-cell segments and forms a blastula, 

 which is invaginated to form a gastrula. The invagi- 

 nated layer forms an endoderm surrounding an enteric 

 cavity, the anterior part of which is developed into 

 a muscular pharynx. The external layer or body-wall 

 develops a muscular ring or collar just behind the position 

 of the pharynx, and near the posterior end of the body are 

 developed a pair of blunt outgrowths, apparently serving 

 as rudimentary organs of locomotion. Between the enteron 

 and the body -wall are numbers of germ-cells like those of 

 previous stages, and in the interior of these a cavity is 

 again developed to serve as a generative cavity. The animal 

 in this stage is called a redia. 



In the sporocyst there is no special or permanent aper- 

 ture from the generative cavity to the exterior through 

 which the rediae can escape : they simply burst through 

 the walls at any point, and the opening so formed contracts 

 and closes up after the redia has passed through. But in 

 the redia there is a definite permanent opening to the 

 generative cavity ; it is situated on one side of the body a 

 little behind the collar, and is called the birth-opening. 



Large numbers of rediae are developed in succession in a 

 sporocyst, and as they become mature they force their way 

 through the wall of the sporocyst and penetrate into the 

 tissues of the host- snail, especially into the liver, feeding on 

 the tissues by means of their mouth and muscular pharynx. 



