144 



HELMINTHOZOA. 



Fig. 72. 



1. Cercaria echinata? of Siebold. 2. Distoma-pupa, 

 or Cercaria in the pupa state after it has cast off its 

 tail and enclosed itself in a mucoid case. 3. The 

 animal proceeding from the pupa a true Distoma, 

 which has penetrated for a short distance into the 

 body of the snail. 4. A "nurse" containing fully- 

 developed Cercarise: v, the stomach. 5. A "parent- 

 nurse " filled with partially-developed " nurses :" v, 

 the stomach. (After Steenstrup.) 



(379.) A Cercaria, supposed by Steenstrup to be the Cercaria echi- 

 nata of Siebold (fig. 72, l), is found by thousands in the water wherein 

 specimens of the large fresh- 

 water snails, Planorbis cor- 

 nea and Limnceus staynalis, 

 have been kept. The body 

 of this species of Cercaria is 

 usually of a more or less 

 elongated- oval form, which, 

 however, it is constantly 

 changing, assuming, during 

 its movements, every out- 

 line, from the circular figure 

 which it has in the fully 

 contracted state, to the linear 

 form that it presents when 

 its body is fully extended ; 

 it is furnished, moreover, 

 with a triangular head, at 

 the apex of which is situated 

 the oral orifice, surrounded 

 with an apparatus of spinous 

 teeth ; and a ventral sucker 

 is visible, situated upon the inferior surface of its body ; while inter- 

 nally traces of viscera are discernible (as represented in the figure), the 

 nature of which is not clearly made out. 



(380.) The swimming movement of these Cercarise is very charac- 

 teristic : in performing it, the animal curves its body together into a ball, 

 by which the head is brought near to the caudal extremity, and at the 

 same time the elongated tail strikes out right and left into various 

 sigmoid flexures. In this way they may be seen swarming about the 

 water-snails in great numbers. After swimming about the snails for 

 some time, they affix themselves, by means of their suckers, to the 

 slimy integument of those animals, and all their movements upon it are 

 readily perceived with a good glass. On examining, with a sufficient 

 magnifying power, a portion of the skin of the snail with several of the 

 Cercariae adhering to it, it will be seen that all the efforts of these 

 creatures are directed to the inserting of themselves deeper into the 

 mucous integument, and to the getting rid of the tail, which is no 

 longer of any use to them as an organ of locomotion ; in this, after 

 violent efforts, the Cercaria at length succeeds, and the now tailless 

 animal assumes so completely the appearance of a Distoma or fluke, that 

 it could not fail of being recognized as belonging to that genus, in case 

 it were met with in this condition in the viscera of other animals. How- 

 ever, it undergoes a further remarkable transformation before it becomes 

 a true Entozoon in the common acceptation of the word. 



