30 THEORY OF THE ORIGIN OF PARASITES. 



eggs of certain Distomidse contained an embryo (Fig. 16) which in 

 shape and ciliation resembled an Infusorian; it was occasionally 

 provided with an eye-speck, and after being hatched swam about like 

 a Infusorian. 1 What a blow was this simple 

 discovery to the earlier theories as to the fate 

 of the eggs of Entozoa. It had certainly been 

 known from the time of Goze that there were a 

 few viviparous Entozoa, but these were in every 

 case thread - worms, whose young so closely 

 resembled the parent form, that they might 

 easily be supposed to attain to their full 

 development without any migration or further 

 change. In the case discovered by Mehlis, 

 FIG. 16. -Ciliated embryo however, the eggs had been laid, and the em- 



rfjkfonostamum capita ^^ entirely uulike their parentj seemed) from 



their eye-specks and coating of cilia, fitted for 

 a free existence. We recall at once the opinions expressed by Leeuwen- 

 hoek and Pallas, and it is quite intelligible that von Nordmann, 

 who first confirmed the observation of Mehlis, remarked that these 

 parasites, instead of originating by spontaneous generation, " always 

 sojourn during their first life-period in water, and subsequently enter 

 the body of some animal, where they lose their eye-specks and 

 become sexually mature." 2 Von Nordmann certainly acknowledges 

 that this sounds " fabulous " in comparison with the generally 

 accepted theory, but, after further reflection, he insisted upon it, since 

 he found in the gut of a Neuropterous larva, three-quarters of a line 

 long, a species of Nematode with a conspicuous red eye, which was 

 found also living independently in water. 



Soon afterwards von Siebold 3 added to these observations the 

 remarkable fact that the ciliated embryo of Monostomum mutdbile 

 (Fig. 17), a parasite of water-birds, sheltered within its body another 

 creature (a " necessary parasite," as it is termed), which so strikingly 

 recalled the " kingsyellow worm " (Eedia), found by Bojanus in pond- 

 snails (Fig. 18), that one might almost believe "that this creature 

 continued to live after the death of its jailor and perhaps grew into a 

 similar form." Unfortunately this idea could not be proved, although 

 its demonstration would have been of the greatest importance. Von 

 Baer had previously shown that these Bedise 4 gave rise, by a develop- 



1 Oken's 7*w, p. 190, 1831. 



2 " Mikrographische Beitrage," Bd. ii., p. 140, Note, 1832. 



3 Archivf. Naturgesch., Jahrg. i., Bd. L, p. 69, 1835. Burdach, " Physiologic," Bd. 

 ii., p. 208 : Leipzig, 1838. 



* Nova Act. Acad. Cces. Leop., t. xiii., p. 627, 1826. 



