THE TREMATODA. 53 



It was well known that this form was not a permanent 

 one, but we were ignorant of the changes which it under- 

 went. As I have been so fortunate as to trace these 

 changes, I will now detail them. 



(a) Cercaria echinata? Siebold. 



(Tab. Ü.) 



I cannot determine with certainty whether the Cercaria 

 which I have figured in tab. ii, fig. 6, be really the C. 

 echinata of Siebold, or a very nearly allied species ; I 

 however, think they are identical, from the description 

 given in Burdach's ' Physiology,' (2d edit. ii. s. 187.) 



This species has of course the usual character of the 

 Cercarice ; a body formed entu^ely like that of a Bistoma, 

 but which is prolonged posteriorly into a long, transversely 

 striated, extensible tail, not a vestige of which is presented 

 by the Distomata. They are found by thousands, and 

 frequently by millions in the water, in which two of our 

 largest fresh-water snails, Planorbis cornea and Limnceus 

 sta^nalis, have been kept. The body is usually of a more 

 or less elongated oval form, which, however, it is constantly 

 changing, assuming, during the continual movement of 

 the animal, every outline, from the circular figure which 

 it has in the fully contracted state, to the linear figure 

 which it possesses when extended. During its various 

 contractions it is more or less evident that a shallow 

 circular depression exists at the anterior part of the animal, 

 which might almost be termed a neck, as it separates that 

 portion which is nearest the mouth from the rest of the 

 body. This depression or neck is partially overlapped by 

 a delicate overhanging collar, by which the head (if the 

 portion beyond the contraction and containing the mouth 

 may be so called) joins the depression. When the crea- 

 ture depresses its head close to the body, this collar stands 

 almost straight out from it, and when the animal in this 

 state is viewed laterally, the collar appears like a small 

 tubercle on each side ; but when it turns itself, or is 



