PARASITES FREE WHILE YOUNG. 



149 



sites after tLey have passed their youth in complete in- 

 dependence, and have all possessed the graceful forms so 

 characteristic of the Naiqjlius and the Zoe. When they 

 first leave the egg, they swim about in freedom, but 

 at length some day the female, thinking of a family, 

 looks out for a neighbour that can give her the assist- 

 ance she requires, fixes herself on his skin, and rapidly 

 develops till she is two or three hundred times as large 



2 



Fig. 30.— Trachcliastes of the Cyprinae. 1, larva, as it leaves the egg ; 2, larva, more 

 advanced ; 3, adult female, attaching itself before and behind to two ovisacs (Nord- 

 mann). 



as the male; her head, her body, and her stomach 

 become of a monstrous size, a part of her head is often 

 anchylosed in the bones of her host ; the lernean 

 remains suspended as a sort of festoon, to which are 

 afterwards joined two ovisacs filled with eggs. Fig. 30 

 'is a lernean of a fresh-water fish, represented at 

 different periods of its existence. 



