22 INTRODUCTION. 



forward from the centre of the animal's head. (Fig. 15 B.) I 

 could readily perceive, that these Cercaria which I was observing, 

 frequently paused in their inspection of the insects, and inserted 

 this weapon into their bodies as they crept over them. This 

 probing experiment, for it w r as clearly nothing else, was repeated 

 again and again, until the larva had discovered one of the soft 

 places between the segments of the insect's body; this being 

 reached, it never moved from the spot, but worked incessantly 

 with its spine, until a way was bored through the soft place it had 

 fastened on. Scarcely was the point of the spine fairly through, 

 ere the supple worm inserted his thin anterior extremity into the 

 wound, widened the opening a little, and by degrees drew in his 

 whole body, which became wonderfully slender under the opera- 

 tion. The tail of the Cercaria was not drawn inside the insect, 

 but remained hanging outside the puncture, being doubtlessly 

 seized and nipped off, by the sudden closing of the wound when 

 the body of the Cercaria had slipped through. Having selected 

 very young and delicate Neuropterous larvae for my inquiry, the 

 transparency of their bodies enabled me to continue to observe 

 the tail-less Cercarice after their entrance ; they forthwith lay 

 still, drew themselves up into balls, and surrounded themselves 

 with a cyst. During the process of encysting, the frontal spine 

 fell off from the body of the Cercaria, and lay apart by its side, 

 but enclosed within the cyst. 1 This weapon, 

 Fig. 16. therefore, undergoes the same fate as the tail of 



these animals, each apparatus being cast aside 

 after fulfilling its intended end. 



The impulse to immigrate and become en- 

 cysted is so strong in all the Cercarice, that 

 their efforts appear to be occasionally over hasty, 

 and perhaps lead them altogether astray. 



I have found in Aselli and Gammari, encysted Cercarice, which 

 in every way resembled those which had passed into insects. 



Fig. 16. An encysted Cercaria armata. a. Oral sucker. b. Ventral sucker. 

 e. Digestive canal which is connected with the oral sucker, d. Urinary organ 

 filled with granular urine, e. Cast off frontal spine which now lies free in the cavity of 

 the cyst. f. Aperture of the urinary organ, which becomes visible after the tail is cast 

 off. g. Cyst in which the tailless Cercaria remains encysted as an asexual Distomum. 



i The observation above detailed (which I have already published in Wagner's 

 ' Handworterbuch,' Bd. ii, p. 669,) can be easily repeated, since the sacs of Cercaria 

 armata are excessively common in our fresh-water snails. 



