Tlie Hot in Sheep. 



53 



consequence of the action of prolonged immersion, aided by the vigorous 

 movements of the contained embryo. 



" The ciliated, free-swimming embryo, at the time of its birth, exhibits the 

 figure of an inverted cone, its anterior extremity, which is broad and some- 

 what flattened, supporting a central proboscis-like papilla. A small pigment 

 spot placed dorsally, and having the form of a cross, is supposed to be a 

 rudimentary organ of vision. After the lapse of a few days the cilia fall off, 

 the embryo then assuming the character of creeping larva3 (planula;). 



"Notwithstanding its abridged locomotive powers, the nonciliated larva) 

 sooner or later gain access to the body of an intermediary bearer, within 

 or upon whose tissues it becomes transformed into a kind of sac or sporocyst. 

 In this condition the larva is capable of developing agamogenetically other 

 larva) in its interior. The sporocysts are highly organised, forming redice. 

 According to Willemoes-Suhm, the redia of Fasciola hepatka lives on the 

 body of Flanorbis marginata. This organised nurse, which is about a line in 

 length, is the Cercaria cystophora of Wagener. The progeny of this redia 

 consist of armed Cercariae, which after a time quit the nurse to pass an in- 

 dependent existence in the water." * 



We may explain that the sporocysts have been also described 

 as Cercarios sacs, and that Steenstrup and others have spoken of 

 the development of Cercariac within these sacs under the term 

 of "Alternation of Generation." 



We insert Steenstrup's definition of the process : " Alternation 

 of Generation is," he says, " the remarkable phenomenon of an 

 animal producing an offspring which at no time resembles its 

 parent, but which, on the other hand, itself brings forth a 

 progeny which returns in its form and nature to the parent 

 animal ; so that the maternal animal does not meet with its 

 resemblance in its own brood, but in its descendants of the 

 second, third, or fourth degree of generation." | 



Many examples of this system of propagation take place 

 among creatures far higher in the scale of organisation than 

 those of which we are now speaking ; but it is unnecessary, in a 

 treatise of this kind, that these should be supplied. We may, 

 however, direct the reader who is desirous of obtaining such 

 information to Steenstrup's work, and also to Professor Owen's 

 on Parthenogenesis, Kiichenmeister's on Parasites,% and Von 

 Siebold's on Cystic Worms. § 



The Cercarice, so called from their caudate form (see fig. 12, 

 page 55), were for a long time considered as Infusoria when 

 found to be floating freely in water, their origin and mode of 

 propagation being unknown until the discovery of Steenstrup. 

 The cercaria-sa.es were designated by him " nurses," and the 

 young cercarixc developed within them "parent-nurses'"' — terms 



* 'Parasites, a Treatise* on F.ntozoa of Man and Animals. 1 By T. Spenoer 

 Cobbold, M.D., F.K.S., F.L.S. London, 1875) 



t 'Alternation of Generations,' by ,). Japetns Sm. Steenstrup, translated from 

 the German by George Busk. London, 1815. 



X Translated by Dr. Lankester. § Translated bj Professor Huxley, 



