THE TREMATOD A. '63 



covering tliem, and tlieir 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 aqueducts or water-passages. 'Fig. 

 4 a represents two of these " nurses" a little magnified ; 

 theii' great difference from the CercaricB, and still more 

 fi'om the fluhes is very apparent. Fig. 4 a shows a 

 similar individual more highly magnified, and also slightly 

 compressed between slips of glass, which renders the vast 

 number of CercaricB within it more distinct. It is not 

 only the outlines of this brood of Cercarics, which are 

 seen in the " nurses' thus compressed, but several of 

 their internal organs as well, as for instance, the forked 

 intestine and the liver. 



As regards the external form of these worms, it must 

 be allowed that it is usually alike in all, unless they have 

 suddenly contracted themselves, in consequence of some 

 external irritation, and owing to which contraction they 

 are sometimes scarcely recognizable from the alteration in 

 their shape. However, the number of these creatures 

 is so great, that they are crowded together on the walls of 

 the water-passages, or in the cellular tissue surrounding 

 the liver, kidneys, &c., and on this account it not unfre- 

 quently occm-s that, even within the snail itself and before 

 the worms have been touched, such contracted misshaped 

 individuals are met with, recognizable only by the oblique 

 projections on the posterior half of the body.* 



The " ntirses" usually present the appearance given in 

 fig. 4 a and b. The body is cyhndrical, 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. Below the head is a sort of collar 

 which abuts upon a depression, which according to the 

 greater or less state of contraction of the animal, is nearer 

 or ftirther from the mouth. When the head is drawn 

 towards the depression, the collar projects straight out 



* Baer has figiired one in this condition. Nova Acta, Acad. Nat. Cur. 

 torn, xiii, tab. 31. 



