ON PARASITES. 17 



individual. Their history has been examined with great care by 

 modern observers, and some departments of the subject have been 

 investigated wit great success. 



Of all the classes, the Cestoidea are best understood. Many inter- 

 esting facts iu connection with the Trematoda are known, but others 

 still require elucidation ; and the same remark is applicable to the 

 H'ematoidea. 



Ail sorts of animals have been opened in search of Entozoa, and 

 when discovered, their anatomy and physiology have been carefully 

 scrutinized, so as to determine their affinities. 



When it is recollected that Helminthology as a science dates from 

 a very recent period, that the metamorphoses of Entozoa are extraor- 

 dinary, and without apparent analogy among the animals inhabiting 

 the outer world, as they were known to the older naturalists, there 

 is abundant reason for satisfaction at the position which this depart- 

 ment of Zoology at-present occupies. The facts discovered are new ; 

 the mind has not habitually contemplated them, hence their due value 

 as yet, may not be accurately determined. In Linnaeus' Natural Sys- 

 tem, 12th edition, eleven species of intestinal worms are described. 

 InEudolphi's Synopsis entozoorum, nearly one thousand are cata- 

 logued. Since his time, some of his species have been corrected and 

 consolidated, but others have been discovered. 



Here as elsewhere, presumption has impeded the acquisition of 

 positive knowledge. Nature has often been interrogated in a wrong 

 spirit. Observers have not invariably manifested a single-hearted desire- 

 for the truth, irrespective of preconceived notions. False impressions 

 acquired by one sided and too hasty observations have not unfre- 

 quently been pertinaciously maintained, with an unfairness highly 

 reprehensible. When Von Siebold established the identity of the 

 seolex of the cystic worms with the head of the tape-worms, he did 

 good service to the cause of science. But his pertinacious mainte- 

 nance of his opinion that the cystic worms were strayed and degene- 

 rate or monstrous cestoids, long stood in the way of the acceptance 

 of the true explanation concerning these two forms. 



Many of the older naturalists, because they could not see certain 

 entozoa spring from eggs, although sexual, considered their origin 

 spontaneous. 



The identity of the seolex of the cystic worm, with the head of the 

 cestoid having been ascertained, an important question yet remained 



VOL. IV. C 



