140 REPRODUCTION. 



the fish are one, because at one stage of embryonic life it is 

 impossible, with the means at our command, to distinguish 

 them. 



36 J. The account we have above given of the develop- 

 ment, the metamorphoses and the alternate reproduction of 

 the lower animals, is sufficient to undermine the old theory 

 of Spontaneous Generation, which was proposed to account 

 for the presence of worms in the bodies of animals, for the 

 sudden appearance of myriads of animalcules in stagnant 

 water, and under other circumstances rendering their pre- 

 sence mysterious. We need only to recollect how the 



Cercaria insinuates itself into the 

 skin and the viscera of mollusks 

 (339, 342), to understand how ad- 



Q 



mission may be gained to the most 

 inaccessible parts. Such beings 



Fig. 145. Fig. 146. occur even in the eye of many 



animals, especially of fishes ; they are numerous in the 

 eye of the common fresh-water perch of Europe. To the 

 naked eye they seem like little white spots (Fig. 145) ; but 

 when magnified they have the form of Fig. 146. 



362. As to the larger intestinal worms found in other 

 animals, the mystery of their origin has been entirely 

 solved by recent researches. A single instance will illus- 

 trate their history. At certain periods of the year, the 

 Sculpins of the Baltic are infested by a particular species of 

 Tsenia or tape-worm, from which they are free at other 

 seasons. Mr. Eschricht, found, that at certain seasons, the 

 worms lose a great portion of the long chain of rings of 

 which they are composed. On a careful examination, he 

 found that each ring contained several hundred eggs, which, 

 on being freed from their envelop, floated in the water. 

 As these eggs are innumerable, it is not astonishing that the 

 Sculpins should occasionally swallow some of them with 



