Natural History of Entozoa. 191 



scientific system of zoological classification would range them 

 in different ranks. If all are comprehended under the term 

 Helmintha or Helminths, three separate classes will foJApw, as 

 in Dr. Cobbold's scheme ; one having no distinct intestine, is 

 appropriately named anenterelmintlia, an appellation which is a 

 simple statement of the fact. More highly organized than 

 these, are others with elaborate internal organs hollowed out of 

 the general tissue, and not occupying special cavities. These 

 are the sterelmintha, or " solid worms," of Owen ; and then 

 we find another set whose organs are situated in cavities, and 

 which constitute the ccelelmintha, or " hollow worms," rounded 

 and threadlike in form. 



The intestinal worms (entozoa) offer very remarkable illus- 

 trations of those methods of reproduction that are allied to the 

 vegetable plan of budding, and which are very common in the 

 lower ranks of animated being. Among the higher forms of 

 animals there is no offspring unless a true egg is fecundated by 

 the action of the male, and the germ it contains gradually 

 grows into a resemblance of the parent form. The superior 

 vertebrates bring forth their young in a tolerably perfect state, 

 the preparatory work of their construction being carried on 

 inside the maternal organism. Lower vertebrates, such as 

 oppossums and kangaroos, give birth to their offspring in a 

 very unfinished condition, and they are suspended to the teats 

 of their parent and left to suck their way to shapeliness in 

 that warm and comfortable nursery, the pouch. Very great 

 are the changes which the embryo of the highest creature 

 undergoes before it is permitted to make its entry into the 

 world ; but they are concealed from the ordinary eye, and only 

 made known to us through the difficult researches which the 

 physiologist carries on. If, however, we turn from the verte- 

 brates to the insects, we come at once upon the exhibition of 

 remarkable transformations effected under our very eyes. We 

 see the worm-like larva become a chrysalis, and the chrysalis 

 in its turn grow into the perfect insect, and thus we have 

 afforded to us the striking lesson that the same individual 

 creature can exist under forms and conditions so different, that 

 nothing but the most rigid demonstration could convince us 

 that the wriggling grub, the quiescent pupa, and the lively 

 flying insect were really the same animal at different stages of 

 its being. Continuing our researches, we find that the pupa 

 state is not in all cases an inactive one, but we recognize in 

 the transformable creatures the offspring of an egg fecundated 

 in the usual way. 



Proceeding still further in the inquiry, the common 

 aphis, or plant louse, affords us illustration of another 

 mode of multiplication; and among infusoria it is common to 



