DISTOMIDiE. 29 



append the results of his inquiries, as recorded in the Schluss- 

 bemerkungen of his admirably illustrated memoir.* 



" (a). — The eggs of the trematoda vary in respect of size, 

 form, and colour, being either famished or not with a lid, and 

 accordingly distinguishable. In the mature condition they 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 moUusks, 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 characters — having become 

 free, grows into a nurse, or forms several nurses within itself. 



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

 organized nurse-condition, others exhibit only a simple kind of 

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

 the same species, depending, in aU hkehhood, upon external causes. 



" (c). — The organized nurses (or redioe, 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 redi^. loose their structure. I did not 

 observe any vascular system. Tailed trematode larvffi {cercarice) 

 as well as redi^ themselves, are developed within the redige, this 

 variation of nurse-contents probably depending on the season. 



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

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

 matode larv£e as are capable of arriving at sexual maturity are 

 furnished with special appendages. 



" (e). — ^When the immature contents of bo.th nurse-forms (i. e., 

 of sporocysts and redise) are accidentally set free, and are situated 



* Trematodenlarven und Trematoden. Von H. A. Pagenstecher, M.D., Heidel- 

 berg, 1867. 



