TEANSMIGRATIONS AND- METAMORPHOSES. 209 



Let us sketch these phases. All the cestodes have eggs, 

 usually in great number, very well protected against 

 external agents. They endure heat and cold, drought as 

 ■well as humidity, resist by means of their envelopes the 

 most violent chemical agents, preserve the faculty of 

 germinating, we will not say for weeks, months, and 

 years, but for centuries. When they first leave the egg, 

 we see an embryo of an oval form, transparent, composed 

 apparently of sarcode, contractile throughout all its 

 extent, and in the middle of which we perceive six stylets 

 arranged in pairs, and which at last move with great 

 rapidity. 



The following is the manner in which, some years 

 since, we described these six hooked embryos produced 

 by a taenia of the frog, which were struggling by the side 

 of each other on the slide of a microscope. " The six 

 hooks are arranged regularly in each individual, and 

 move exactly in the same manner. They are very slight, 

 and of nearly half the diameter of the embryo. Two 

 occupy the median line, and unite like a single stylet ; 

 these are nearly straight, and a little longer than the 

 others. They only move backwards and forwards. Then- 

 action is like that of the parts of the mouth in certain 

 parasitical crustaceans, the Arguli, when they endeavour 

 to pierce through the tissues. They are in continual 

 motion to and fro. The other four hooks are similar to 

 each other, and differ from the first in the point, which 

 is curved into real hooks. They are arranged two and 

 two, to the right and left of the first, so that they all 

 meet at the base. Their movements are not the same as 

 those of the two first ; they remain almost fixed at the 

 base, while they describe a quarter of a circle at the 



