61 



nished with six hooks, arranged in three pairs, one 

 in front and one at each side (PL II. fig. 1, a). Those 

 in front bring themselves together into a point, and 

 move backwards and forwards ; the two lateral ones 

 are armed at the apex with a pair of hooks, and move 

 sideways. According to Van Beneden, progression is 

 accomplished by bringing the two centre hooks into a 

 wedge-like shape, thrusting and twisting, while at the 

 same time it assists itself with the two lateral pairs of 

 hooks, and with them pushes itself onwards, "just as 

 a person who wishes to spring out of a low window 

 rests his elbows against the window-frames, and drives 

 himself forward with a spring ; " but Leuckart thinks 

 that the movements are by no means effected right 

 and left in all the hooks in the same direction, but 

 that, " whilst the two lateral pairs move downwards 

 from the vertex in the lateral plane nearly simulta- 

 neously, this movement can only take place somewhat 

 later in the central pair, and then in the median 

 plane." Thus sometimes one pair of hooks (the cen- 

 tral one) appears to rest, and to hold the embryo in 

 its place, whilst the two lateral pairs are in motion ; 

 and sometimes the central pair appears to be active, 

 whilst the other pairs hold the embryo and preserve 

 it from slipping back, which certainly appears most 

 in accordance with nature (Kiichenmeister). Which- 

 ever the method of boring be, the embryo quickly 

 penetrates through the walls of the intestine into the 

 tissues beneath, where it takes up its abode. This 

 may be done entirely by the active method of migra- 

 tion, viz., boring, more particularly in those embryos 



