48 ANIMAL PARASITES. 



With this we come to the end of the passive migration of the 

 eggs of the Cestodea, and of the brood in the eggs. 



III. The destimj of the six-hooked brood when set free. 



This subject must be divided into two sections : 



1. What is the fate of this hrood until it reaches its settled 

 dwelling place external to the intestinal canal ? 



2. What becomes of it in this dwelling place ? 



1. As soon as the embryo has become free in any part of the 

 body (especially, therefore, in the stomach) of an animal which 

 suits it, the instinct of active migration awakens in it. For this 

 purpose, according to Van Beneden, it brings together the 

 central pair of its embryonal hooklets like a wedge (Tab. I, 

 fig. 5), and at the same time, by thrusting and twisting, begins 

 to force them forward. Having in this way made a little pro- 

 gress, it assists itself further with, the two lateral pairs of hook- 

 lets, and pushes itself forward therewith, 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 swing. 



According to Moller, the central spines, which are the longest, 

 and also straight, move exactly as described by Van Beneden; they 

 lay themselves together in the form of a stile and only move 

 forwards and backwards. The other two lateral pairs have small 

 hooks at the apex. All the six have their roots directed towards 

 the same point, by which means they pretty nearly touch each 

 other at the base. Moller, however, compares their movements 

 with those of a watch, on the face of which he imagines three pairs 

 of hands all pointing to twelve. The central hooks are driven 

 directly forwards, whilst the other pairs go back, the one to three 

 and the other to nine, so as at last to form a right angle with the 

 central ones. I have always described the movements of these 

 hooklets according to Van Beneden, but I have never been able 

 to arrive at a perfectly clear idea of the movement of those 

 embryos in which the central spines do not form straight stiles, 

 but are also curved in front in the form of a claw, although 

 perhaps somewhat less strongly than the lateral hooks. Such 

 spines occur especially amongst the large-hooked Tcejiice of birds, 

 as Avell as in the small-hooked embryos of the Tanice of the 

 human subject. Leuckart is of opinion that the movements are 



