ALTERNATION OF GENERATIONS. 113 



ternal organs, though there is a certain gradation observ- 

 able in this respect. In some there is a distinct head, with 

 a mouth opening into a sort of blind gullet, representing 

 perhaps the commencement of an alimentary canal, and 

 posteriorly the tail is marked off from the cylindrical body 

 by a pair of processes like rudimentary limbs ; others 

 again are merely elongated sacs, not always endowed even 

 with contractility. The former Filippi terms Redice, the 

 latter Sporocysts* 



In the farther course of development, the redia or sporo- 

 cyst forms in its interior a number of growing points, which 

 assume the form of Cercarice, or microscopic tadpoles. The 

 parent cyst in time gives way, under the pressure caused by 

 the growth of its progeny, which are thus set at liberty, and 

 boring their way out of the snail by hooks on their heads, 

 they swim about for a time freely in the water. Their re- 

 lations and farther progress are thus graphically described 

 by Prof. Owen. " No sexual organs exist in these Cercariw, 

 any more than in their animated ' coat,' the Gregarina, or 

 in their ciliated ' great coat,' the Infusorial embryo. After 

 the larval Cercariae have passed some time in the water, first 

 creeping and then swimming about with great restlessness, 

 they either enter directly the body of the waterfowl,^ or 

 bore their way into some aquatic insect, or they may fail in 

 both these instinctive efforts, and remain in the water. In 

 any case they undergo a metamorphosis. The Cercaria 

 gathers itself up into a ball, and exudes a mucous secretion 

 from its surface, which soon hardens ; and since the worm, 

 inside this mucous mass, turns round without stopping, it 



* Siebold's Compar. Anatomy, 118, n. 7. Huxley, in Medical Times, 

 XIII., 133-134. Busk's Translation of Steenstrup on Alternation of 

 Generations, (Ray Soc. Pub.), p. 63. 



f The Monostomum mutabile, a Trematode, whose development is 

 quite parallel to that of Distoma, has for its habitat the eyelids of ducks 

 and other waterfowl. 



