146 HELMINTHOZOA. 



(384.) The " ntirses " usually present the appearance of the figure 

 given above (fig. 72, 4). The body is cylindrical, and is furnished in 

 most instances with a spherical contracted head, which includes an oral 

 cavity with very muscular walls and a small circular mouth. At some 

 distance posterior to the middle of the body are situated the two cha- 

 racteristic oblique processes, which, as well as the part of the trunk 

 posterior to them, are simply local dilatations of the cavity of the body. 

 Of internal organs, there is only to be seen an undivided sacculated 

 stomach (y), very small in proportion to the size of the animal. 



The whole remainder of the very large body is filled with the brood 

 of Cercarice. In the instance figured above (fig. 72, 4), all the embryos 

 have simultaneously reached their full development, which is but seldom 

 the case, since, in the same individual, Cercarice are found in all stages 

 of development. 



(385.) Some doubt exists as to the mode in which the Cercarice quit 

 their " nurses," since it has been observed, under the microscope, that 

 there are two places where they come away, viz. from each side of the 

 body, at a depression under the collar, and from the abdominal surface, 

 between the two oblique processes : but they escape from the latter 

 situation only when the animal has been slightly compressed between 

 the glasses ; and from the former, on the contrary, when no pressure at 

 all has been employed. 



(386.) It next remains to trace the origin of the "nurses" them- 

 selves. Siebold (who did not regard these as independent animals, but 

 only as living organs of generation, "germ-sacs") expresses his surprise 

 at seeing them developed from germs which are always contained in 

 other creatures having the same outward appearance as themselves ; 

 and Steenstrup saw, with like astonishment, that it constantly occurred, 

 in some of the snails taken from the same places as the others, that 

 they harboured only Entozoa which had the outward form of the 

 " nurses" but which, instead of Cercaria3, contained a progeny consist- 

 ing 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 

 inhabited by "nurses" whose progeny were true Cercarice:, it cannot, 

 therefore, be doubted that it is normal for the " nurses" to originate in 

 creatures of similar appearance to themselves, and which are thus the 

 " nurses" of " nurses" These "parent-nurses" however (fig. 72, 5), 

 notwithstanding their great resemblance, were not difficult to be distin- 

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



stagnalis from small stagnant pools that have been exposed to the sun ; the worms 

 will be very readily found. They are situated not so much in the viscera themselves 

 (the liver and reproductive organs) as in the membranes covering them, and their 

 long bodies will be found half-floating as it were in the fluid which occupies the 

 space between the organs, and which appears to be pure water entering through the 

 water-canals. 



