184 EMBRYOLOGY 



contrary, this sta,<?e of development happens in the wai'm 

 season, then creatures of another shape are developed from 

 the o-erm cells : the tailed Cercaria- i¥\^. 89 E [contained 

 individuals] and F). In other cases the Cercaria3 arise only 

 ill the Redia? of the second generation. 



The mode of origin of tlie Cc>cariip has been thoroughly studied by 

 ScHWARz (No. i)). As has ah-eady been mentioned, this investigator finds in 

 their origin a great resemblance to the development of the embryo. The 

 woruZrt -like heaji of cells which arose from the germ cell is further 

 developed in such a manner that there are dilferentiated a peripheral 

 cell-layer, a central compact mass of cells, and a layer between the two. 

 The first supi^lies the dermal layer, which is to be considered as a meta- 

 morphosed epithelium ; from the central mass arise the genital organs, 

 whereas the intermediate parts of the embryonal tissue (the " meristem " 

 of ScHWAEz) give rise to the other organs. Anterior to the central cell- 

 mass a number of cells are arranged in a regular manner. This is the 

 fundament of the intestine, which later becomes hollowed out and con- 

 tinuous with the two branches of the intestine, which have arisen in the 

 same way. The central part of the excretory apparatus is also formed 

 by means of such a regular arrangement of cells in the posterior part of 

 the body. The dermo-muscular layer and the fundament of the nervous 

 system arise nearer the periphery. The remaining part of the " meri- 

 stem " becomes the parenchymatous tissue of the body. 



The Cercarla already exhibits to a certain extent the 

 organization of the adult Dutownm^ e.g., in the presence of 

 an anterior sucker and one situated on the ventral side 

 (Fig. 89 F). In the centre of tlie former lies the mouth, 

 which leads into tlie muscular pharynx, and thence into the 

 forked intestine. The oesophageal ganglion, with the two 

 lateral stems, and also the bipartite excretory system are 

 present. But a long muscular tail is attached to the 

 posterior portion of the body. In this condition the Cercaria 

 leaves the Redia through the birth aperture, which lies at 

 the anterior end (Fig. 89 E, G), and seeks an escape by work- 

 ing its way through the tissues of the host by means of its 

 suckers and tail. Its free life in the water lasts for only a 

 shoi-t time. It soon attaclies itself to plants which arc 

 found at the water's edge. It casts off the tail, and secretes 

 about itself a cyst. A large number of glands which lie on 

 either side in the body of the Cercaria, and which give a 



