58 DEVELOPMENT OF 



(fig. 7.) This has inclmed Siebold to doubt whether a 

 proper pupa case is formed. I think the animal really 

 casts a very delicate cuticle ; and in this opinion I am 

 supported, chiefly by observing that after the assumption 

 of the pupa state all the internal organs are more dis- 

 tinctly seen ; for instance, through many pupa cases, the 

 formation of which I have watched under the microscope, 

 the above-described circlet of spines in the collar might 

 be readily seen, which before the pupa condition had 

 been assumed were distinguishable only with great diffi- 

 culty. The position of this circlet of spines necessarily 

 varies very much according to the position and motions 

 of the animal within the pupa case. Compare the figures 

 7 a, h, c, the last of which represents a pupa under the 

 highest power of my microscope. 



The number of pupa which may be found assembled 

 on the cuticle, and especially on the mantle over the back 

 of the snail, is very remarkable ; and I have several times 

 observed the skin of these animals, in the greater part of 

 its extent, as thickly set "with the pupse as is represented 

 at fig. 7 d. It must, however, be remarked that this 

 pupa condition is also assumed before the eyes of the 

 observer, in water and on a slip of glass, without the ad- 

 dition of any of the mucus or other substance from the 

 snau, and it takes place likewise in the interior of the 

 snail (when Cercarim occur there in a way which we shall 

 soon see) in the water-chambers or reservoirs of water, 

 betweeen several of the organs of the snail, in which the 

 Cercarice are frequently obliged to swim, instead of in 

 the water external to the animal. 



This was the extent of our knowledge as regarded 

 these creatures. 



The expression of the helminthologist Siebold, that 

 " what becomes of these pupa Cercarice is at present an 

 enigma," applies, as far as my knowledge of science ex- 

 tends, as well to these as to all the other species of Cer- 

 carice. I hasten, therefore, to solve the riddle. How 

 long they remain in the pupa state, I am unable to say 



