4 ANIMAL PARASITES. 



without a passive and active migration of the embryos and imma- 

 ture young. 



This migration itself is passive during the embryonal state, 

 and as Ion"; as the embrvos are still enclosed in the esr^-shells or 

 egg-envelopes, during which period they usually emigrate once, 

 passively, into the external world, generally with the excrements 

 of the hosts, 1 of their parents, and then again, passively in general, 

 into the intestinal canal of the animal in which they are to acquire 

 a higher development. From the latter moment commences 

 their active migration, by which they seek the situations, usually 

 exterior to the intestinal canal, in which they are to undergo 

 their metamorphosis into the next higher step or steps of deve- 

 lopment, which is generally accompanied by an encysting process. 



Finally, as a general rule, at least as regards the trematode and 

 cestoid worms, they must afterwards migrate passively once more 

 into the intestine of the animal in which they are to attain 

 maturity ; that is to say, they must pass with the food of their 

 new and final host into its intestine. 



The general etiology proves that a spontaneous generation 

 {generatio cequivoca) never takes place, but that there is always a 

 reproduction from sexual parents, either directly or by indirect 

 channels. 3 



The general pathology shows that the worms do not effect their 



1 [" Host" is a literal translation of the German " Wirth," and although not perhaps 

 previously used in the ahove sense in the English language, I have adopted it to prevent 

 a somewhat tedious circumlocution. E. L.] 



2 Very recently we have obtained much new information relating to the theory of 

 reproduction. In the first place, the discovery of the male generative organs in the 

 earth-worms by Hering, of Leipzig, has proved these animals to be hermaphrodite ; and 

 we have perhaps some reason to hope that we may find this hermaphrodism more widely 

 diffused over other sections of animals than has hitherto been the case ; for example, 

 amongst the insects. Whether there really are creatures amongst insects which pro- 

 duce young without any sexual union, in the manner of nurse-production, is a question 

 of the day raised by Von Siebold's theory of Parthenogenesis in Psyche helix, Solenobia 

 clathrella and lichenella, Bombyx mori, and Apis mellifica. However, even in this case 

 it will be seen that there is no reference to a generatio cequivoca. (See Hering, in Siehold 

 and Kolliker, ' Zeitsch. fur Wiss. Zool.,' viii, p. 400, and Von Siebold's ' Parthenogenesis,' 

 Leipzig, 1856, transl. by Dallas Van Voorst, 1857.) Czenkowsky's statements, which 

 amount to a true generatio cequivoca, however, appear to us to be quite incredible ; he 

 professes to have seen swarm-spores furnished with two cilia produced by a peculiar 

 metamorphosis from the starch-grains of decomposing potatoes. Must not this isolated 

 fact be a mistake ? For our part, we believe that the theory of the generatio cequivoca 

 cannot be considered as supported or proved by this statement. 



