TREMATODES : DEVELOPMENT 



227 



segmentation cells of the ovum is only imperfectly known. They have 

 a cuticular epithelium (fig. 129) entirely or partly covered with cilia, 

 beneath this a dermo-muscular tube composed of circular and longi- 

 tudinal muscles ; also, a simple gut sac with an oesophagus, occa- 

 sionally also with pharynx, salivary glands and boring spine, also 

 a cerebral ganglion on which, in some species, there are eyes 

 (fig. 131, a). As to the excretory organs, they are represented by two 

 symmetrically placed terminal flame cells, with excretory vessels 

 opening separately ; there is a 

 more or less ample (primary) body 

 cavity between the parietes of the 

 body and the gut ; from the cel- 

 lular parietal lining of this cavity 

 single cells (germ cells) become 

 free (fig. 131, <?, 6), and become 

 rediae or cercariae. 



[The germ cells of the mira- 

 cidium and the germ balls of the 

 sporocyst arise, according to some 

 observers, by further division of 

 undifferentiated blastomeres ; ac- 

 cording to others from the cells of 

 the lining wall of its body cavity. 

 It is from these free germ balls 

 that the redia stage is developed. 



[In the germ ball or morula 

 appears an invagination, giving 

 rise to the cup-shaped gastrula 

 stage. This elongates and forms 

 the REDIA (fig. 131, c). 



[In the interior of the redia 

 cells are budded off and develop 

 into gastrulae, as in the case of the 

 sporocyst. These become a fresh 

 generation of rediae or give rise 

 to the third stage (CERCARIA).] 



(2) The SPOROCYSTS, on the contrary, which are produced direct 

 from the miracidia, are very simple, as all the organs of the latter 

 disappear, even to the muscles and excretory organs, during or after 

 penetration into the intermediate host, whereas the budded and still 

 budding cells of the wall of the (primary) body cavity continue to 

 develop rapidly and form germ balls. The sporocysts when fully 

 developed have the appearance of tubes or fusiform bodies with 

 rounded edge; they are frequently of a yellow colour. Their length 



FIG. 132. Young FIG. 133. Older 



redia of Fasciola he- redia of Distoma 



patica.) with pharynx echinatum, with ru- 



and intestine, with a dimentary intestine 



circular ridge an- i. ; cercariae, c. ; germ 



teriorly and a pair balls, b. ; and birth 



of processes poste- pore, g. Magnified, 

 riorly and masses of 

 cells (germ balls) in 

 the interior. Mag- 

 n i fi e d. (From 

 Leuckart.) 



