18 ON PARASITES. 



for investigation, viz : whetber the caudal vesicle of the former was 

 to be regarded as the result of circumstances, or a stage in the ordi' 

 nary development of the perfect animal from the six hooked embryo. 

 It will be seen that this is a part of the general question, how far ex- 

 ternal circumstances modify the growth and development of an or- 

 ganism. Do types change ? A question of the highest importance, 

 and which lies at the base of all physiological science. It bears upon 

 the important subject of classification, without which, zoology and 

 botany would be a mere jumble, what in fact chemistry was, until 

 the discovery of homologies. It therefore may justly be considered 

 before proceeding fco the classification of the entozoa. 



An extended survey of the animal kingdom establishes positively the 

 fact that there is a progression which is quite regular, from the sim- 

 plest infusory animalcule, up to man. Cuvier's observations prove 

 that animals after the precise types of the present, were in existence 

 4,000 years ago, and that the fossil animals were of diiFerent species. 

 If present types have existed so long and fossil ones have perished 

 with the cessation of the conditions necessary for their maintenance, 

 the conclusion seems irrefragable that types are constant. This con- 

 stancy is preserved through the medium of a continual succession of 

 individuals, that find suitable conditions always for their development. 

 When those conditions terminate, the succession also terminates, and 

 with new arrangements of matter appears a new type that goes on 

 as before. It may be here observed that all man's efforts at the so 

 called improvement of useful plants and animals have merely resulted 

 in modifications of growth, and not in development. 



I shall now give the classification of entozoa as it is generally 

 adopted at present. 



Being the expression of actual fact it is reliable. The general 

 characteristics of the species of entozoa infesting at some time the 

 human subject will then be given, and afterwards, their embryology. 



Entozoa (Helminthes.) 

 Class I. 



No intestines 



No mouth or anus 



Sexes united ... \q^^^^ i. Cestoidea. 

 Soft integument J 



•^exessepara e.. | Qj.^^^ 2. Acanthocephala. 

 Soft integument J 



