EEPTILIA 453 



contain a ciliated or a non-ciliated embryo of unequal growth, 

 this embryo partly increasing in size even after its birth. In 

 various conceivable ways the eggs themselves, or the embryos 

 which have quitted their shells, arrive in and upon the bodies of 

 mollusks, where they are consequently found. In this situation 

 the egg opens, or the ciliated covering decays, and the contained 

 motionless germ which in itself offers no distinctive charac- 

 ters having become free, grows into a nurse, or forms several 

 nurses within itself. 



" (b). Whilst some of the trematodes display a highly 

 organised nurse condition, others exhibit only a simple kind of 

 germ-sac. Both forms, nevertheless, appear to occur in one 

 and the same species, probably depending upon external causes. 

 " (c). The organised nurses (or r edits, as they are termed) 

 have a mouth and a strongly marked muscular oesophagus, 

 which is continued into a short or prolonged, single, blind 

 intestine, or the latter may be double. The expulsion of 

 animals developed within them I have only seen to take place 

 through an opening at the hinder extremity. Old rediaa lose 

 their structure. I did not observe any vascular system. Tailed 

 trematode larvas (Cercarice), as well as redise themselves, are 

 developed within the redise, this variation of nurse-contents 

 probably depending on the season. 



" (d). No independent new germ-sacs are developed within 

 the simple unorganised germ-sacs (sporocysts), and only such 

 trematode larvae as are capable of arriving at sexual maturity 

 are furnished with special appendages. 



" (e). When the immature contents Of both nurse forms 

 (i.e. of sporocysts and redise) are accidentally set free, and are 

 situated within the organs of nutrition of the living host, then 

 they appear prepared to develop .themselves anew into nurse 

 forms ; and, moreover, cercarise whose development has not yet 

 attained a definite stage and even their tails also appear to 

 enjoy a similar capacity. Some nurses are likewise capable of 

 multiplication by division and budding. 



" (/) Some germ-sacs have the property of developing 

 within themselves cercaria-like larvae which are different from 

 the true cercariae from whose body the development of a 

 distoma may take place, while their single or double tail-like 

 appendages in all cases develop anew into germ-sacs. To this 

 class belong Bucephalus and Distoma duplicatum. 



" (g). All the cercaria at present known are destitute of 



