LIFE HESTORY OF THE LIYEK-FLUKE. 113 



As soon as the embryo begins to bore the head-papilla 

 becomes longer, conical, and pointed. The embryo spins round 

 on its axis, the cilia working vigorously and pressing the 

 embryo against the surface of the snail. This pressure is in- 

 creased by the body of the embryo being alternately drawn up 

 and then suddenly extended. As the papilla sinks further into 

 the tissues of the snail, it becomes longer and longer until it 

 may reach five times its original length (Plate II, fig. 6), and 

 the tissues of the snail are forced apart, as if by a wedge, 

 leaving a gap through which the embryo squeezes its way into 

 the snail. 



The embryo appears to exert an instinctive choice in select- 

 ing the host into which it enters. It is conceivable that the 

 tissues of Limnseus truncatulus are softer than those of 

 other molluscs, and that the embryo is able to bore its way 

 into the former whilst it cannot do so into the latter. But 

 from the greater eagerness which the embryo exhibits when 

 placed on a slide with Limnseus truncatulus I do not think 

 that this is the correct explanation. Moreover, the tissues of 

 such snails as young specimens of Physa fontinalis or Lim- 

 naeus palustris appear to be quite as soft as those of L. 

 truncatulus, and yet if a quantity of embryos are placed in a 

 vessel containing equal sized examples of the three species just 

 named, it is found, on subsequent examination, that whereas 

 each L. truncatulus may contain fifty or more intruders, the 

 other snails are quite free from them. The most probable 

 explanation seems to be that there is some difference in the 

 nature of the secretion of the surface of the body in these 

 snails, which is sufficient to serve as a guide to the instinct of 

 the embryos. 



But, although the embryo instinctively chooses the snail in 

 which its further development is possible, it does not always 

 make an equally happy selection of the part of the snail into 

 Avhich it enters. I have found as many as a dozen embryos 

 embedded in the substance of the foot of a L. truncatulus, 

 such a place of course is unfavorable to further development, 

 but they may remain alive there for two or three days. Once 



VOL. XXIII. NEW SER. H 



