76 DEVELOPMENT OF 



embrace its root are of somewhat firmer consistence than 

 the rest of the trunk, and appear during the continual 

 motion of the animal to be used by it as a pair of stumps 

 or feet to push itself forwards with. The tail cannot ever 

 be extended so far as to cause the disappearance of the 

 depression in which it is inserted, or of the lateral folds ; 

 its internal cavity reaches as far as the body, but I have 

 not been able to trace it further. The whole animal is 

 covered with an extremely delicate transparent cuticle, be- 

 neath which the skin, which is highly glandular, secretes a 

 viscid, yellowish-white slime, which hardens under the 

 water, and is of considerable importance during the fur- 

 ther development of the Cercarice. 



The fact that this Cercaria, like the former, assumes a 

 pupa state, has been already observed by Siebold, who 

 has described the phenomena attending that change, in 

 Burdach's ' Physiologie,' t. ii. I will now only add, that 

 it crowds about the water snails in much larger swarms 

 than the preceding species, and attaches itself to their 

 uncovered portions in such numbers as to give the sur- 

 face a flocculent appearance, and that after crawling 

 about for some time, it assumes the pupa state. It is 

 during this creeping about on the cuticle of the snail that 

 the Cercaria exhibits the thrusting or piercing action 

 which is seen to attend its motions on the object-stage of 

 the microscope. Attaching itself by means of the abdo- 

 minal sucker, and pushing itself as it were by the pro- 

 jections on each side of the root of the tail, it thus thrusts 

 the fore part of the body and the spiculum placed on its 

 anterior extremity, with considerable force against the 

 skin of the snail, which is in this way apparently per- 

 forated, for immediately afterwards the Cercaria is seen 

 to be sunk or buried in it, and covered with its slime. If 

 the tail has not previously been detached by the vigorous 

 movements during the alternate attachments and creeping 

 about of the Cercarice, it now becomes so ; and since by 

 this separation of the tail its internal cavity is divided, a 

 new opening is produced at the posterior end of the 



