190 ENTOZOA. 



larva (Nurse, Sporocyst, or Reclia, as tlie case may be). What- 

 ever be the full significance of these internal developments, we have 

 at least satisfactory evidence that the complete and free embryo is 

 a globe-shaped animalcule, having the anterior third or cephalic end 



Fig. 40. — Free embryo of Distoma lanceolatum ; highly magnified. — Leuckart. 



of the body covered with cilia, and armed with a central boring spine. 

 In consequence of this limitation of the ciliated covering, its swimming 

 movements are less vivacious than those of the embryo of Fasciola 

 liepatica ; it will, therefore, probably take up its residence in a less 

 active host than that chosen by the embryo of Fasciola, selecting one 

 of those molluscs which either move slowly or are prone to keep at 

 the bottom of the water. The mature eggs have a length of gls to 

 5I3 of an inch, and a breadth of ^3'. The long diameter of the free 

 embryo varies from 9I0 ' to ^' , the transverse diameter being j^". 

 Whilst the embryos were still in the Qgg Leuckart could see no 

 cihary motion ; with most observers, both the ciliary apparatus and 

 the boring spine appear at this stage to have altogether escaped 

 observation. 



The further transformations of the embryo and larvae of 

 Distoma lanceolatum are not known with certainty, but our conjec- 

 tures on this score cannot be far removed from the actual truth. 

 We shall probably find that the embryo changes into a Sporocyst, 

 the latter producing, directly or indirectly, a colony of armed cer- 

 carian larvae provided with tails. These higher larval forms may 

 again inhabit molluscs, after a longer or shorter period of active 

 wandering, being subsequently transferred along with their hosts 

 into tlie intestinal canal of herbivora, and, in a few cases (probably 

 along Avith watercresses) into the human stomach. Leuckart 



