126 EXPLANATION OF 



they will attain a form different from that of the 

 animal in which they are situated (or fostered). 4 h 

 contains fully-developed embryos, which exhibit the 

 long tail and an internal, bifurcated digestive organ ; 

 they fill the whole cavity of the body of the " nursing 

 animal" so that the saccular digestive organ of the 

 latter is reduced into a small organ marked v. 4< a 

 completed " nurses" slightly magnified. 



Figs. 5, 6, 7, 8. The third generation of tlie Distoma ; 

 the embryo, larva, pupts, and perfect Distoma. 



Fig. 5. The forms gradually assumed by the young 

 within the ''■nursing" animals, whilst from germs 

 they are formed into embryos ; CercaricB, Distoma 

 larva ; figs. 5 a — e spherical germs, which are 

 elongated {g — h) and exhibit a trace of life {i) whilst 

 a tail begins to be distinct from the body. In 

 figs, k and / the form of the Cercaria is completed, 

 and several of the internal organs are distinguish- 

 able. Fig. m unborn Cercaria viewed from the 

 side. 



Fig. 6. A Cercaria which has quitted its " nurse" and 

 swims free in the water. The outward form of the 

 body corresponds with that of the Distoma, as also 

 do the internal organs, but is wanting the tail 

 (vide fig. 8). The head and the collar situated be- 

 neath it correspond with the same parts in the two 

 preceding generations of " nurses" and ''parent- 

 nurses" (vide fig. 2 and fig. 4). The lateral pro- 

 cesses in these are the lateral portions of the poste- 

 rior part of the body surrounding the root of the 

 tau ; the tau is one and the same organ in all the 

 three generations. 



Fig. 7. Distoma pup{S or Cercaria in the state of pupae 

 or when they have cast off the tail and are sur- 



