THE TREMATOD A. 69 



all these young individuals the skin was readily seen to 

 be composed of glandular, circular, nearly equal-sized 

 cells or vesicles. 



I will now explain this origin of the very young 

 " nurses." Siebold expresses his surprise at seeing them, 

 (which he does not look upon as independent animals, 

 but only as living organs of generation, and with Bur- 

 dach designates as "living germ-sacs") developed from 

 germs, which are always contained in other creatures 

 corresponding with them in outward appearance {germ- 

 sacs) ; but he seems to have seen this very seldom. I had 

 certainly had thousands of " nurses' under the micro- 

 scope before I saw this phenomenon, and I freely confess, 

 that for a long time I believed Siebold's observation to 

 be founded on some illusion. In July, August, and Sep- 

 tember, I also saw no instance of it ; but in the winter it 

 constantly occurred in some of the snails taken from the 

 same places as the others, that they harboured only ento- 

 zoa, löhich had the outward form of the " nurses^' hut 

 lohich contained a progeny consisting of actual " nurses^ 

 in all stages of development. This was the case only in 

 some, and those rather young snails, whilst all the others 

 were at the same time inhabited by " nurses," whose 

 progeny was true Cercaria, and I, consequently, cannot 

 doubt but that it is normal for the " nurses' to originate 

 in creatures of similar appearance, and which are thus 

 the " nurses" of " 7iurses." These "parent nurses" 

 {abaltrices) notwithstanding their great external resem- 

 blance, were not difficult to be distinguished from the 

 common ones ; the stomach, for instance, in the full- 

 grown ''parent nurses" filled with their progeny of em- 

 bryo " nurses" was longer and wider than in any even of 

 the youngest " nurses" (compare fig. 2 a and 2 h with 

 4 c and 4 d^ 



In figs. 2 c and 2 d are seen the ''parent nurses" in 

 their younger form. The embryo " nurses" are contained 

 only in the posterior part of the b©cly, between the two 

 lateral processes, which is the same situation as that 



