28 ON PARASITES. 



Having briefly given the prominent characteristics of the Nematode 

 VForms infesting the human body, I proceed to consider the embryology 

 of the entozoa, especially the Cestoids. 



Embryology of Entozoa. 



These beings vrhich are so universally distributed that there is hardly 

 an animal exempt from their attacks at some time in the course of its 

 life : whence do they come ? how do they develope and how multiply ? 

 Are they the spontaneous result of the concurrence of physiological 

 and pathological conditions in which they are produced ? or do they 

 originate from parent organisms in the usual way of generation that 

 obtains among the higher animals ? If so, how far do surrounding 

 circumstances affect their growth and development ? The six-hooked 

 embryo of a Taenia gets into a serous cavity or into cellular tissue and 

 becomes a vascular worm, a Taenia head or larva with a caudal vesicle, 

 instead of the strobila which should develope from that head larva or 

 scolex, were it in the alimentary canal. Is that vesicle the result of the 

 abnormal conditions in which this strayed embryo has fixed itself? 



A cercaria swims freely in the water without sexual organs, digestive 

 cavity, or aught but a propulsive tail ; when transferred to the interior 

 of a mollusk it so radically changes itself that analogy is lost between 

 the two forms that it has assumed. Is this change due to circum- 

 stances? 



Such questions as those necessarily arise in entering upon the study 

 of helminthic embryology. And it is at once seen that they are of the 

 highest importance, not only in regard to these worms themselves, but 

 also for their bearing upon high principles of general physiology. A 

 suitable reply can only be made to them by extended observation. By 

 gathering facts for embryology from all parts of the animal scale, a 

 rational scheme of the subject can be made. Such points as are 

 obscure at one part of the animal scale may be explained by such as 

 are of a similar nature, but in more obvious relations at another part. 

 Positive knowledge of the higher animals rests upon and illustrates the 

 obscurities of the helminthic worms, and these in turn bear upon the 

 study of the creatures that rank above them. 



The mystery which enshrouded the embryology of the higher 

 animals has been well cleared up. With infinite labor a connected 

 series of observations upon the subject has been made, from the con- 

 course of the zoosperm and ovule to the evolution of the perfect animal. 



