56 DEVELOPMENT OF 



is usually reticulated on the surface. Through the centre 

 of these tubes there passes a slightly narrower canal, which 

 becomes very small towards the extremity of the tail. 



The existence of the same layers in the body itself of 

 the Cercaria, can easily be demonstrated ; but the trans- 

 versely striated layer is here not so much developed. 



The swimming movement of these Cercarice is very 

 characteristic; 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 in innumerable 

 sigmoid flexm-es, so that sometimes it presents the appear- 

 ance of a figm-e 8 behind the animal. In this way they 

 may be seen swarming about the water snails in great 

 numbers. The reason for their keeping chiefly in the 

 neighbom-hood of these animals is self-evident on placing 

 a few snails in a glass of water containing a number of 

 the animalculse. After swimming about the snails for 

 some time, they affix themselves by means of their suck- 

 ers to the slimy integument of those animals ; and all 

 their movements upon it are readily perceived with a 

 good glass. 



They frequently detach themselves, roll about in the 

 water in their peculiar way, then return and reaffix them- 

 selves and remain still, attached only by the sucker, and 

 extend both the anterior part of the body and the tail in 

 the same direction; in this position, from the great 

 number affixed at the same time, they give the snail a 

 sort of flocculent appearance. This quietude ordinarily 

 lasts but a very short time ; they soon begin to move 

 about with a leech-like motion in all directions on the 

 surface of the snail ; but after they have wandered about 

 in this way for a short time, as if to set themselves right, 

 they attach themselves again by the sucker, and remain 

 more quiet. On examining with a sufficient magnifying 

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

 the Cercarice adherent to it, it will be perceived, that all 

 the efforts of these creatures are directed to the inserting 



