ON PARASITES. 



19 



Class II. 



Intestine terminating in a csecal extremi- 1 



ty, without amis I Order 3. Trematoda. 



Sexes united J 



Class III. 



A perfect intestine 



' Mouth situated on 

 the ventral surface 

 provided with four 

 retractile hooks. 



Sexes distinct ■ Mouth at or near 



the anterior ex- 

 tremity and with- 

 out retractile hooks 



Order 4. Acanthotheca. 



Order 5. Nematoidea. 



It will be observed that sexuality is an essential characteristic of 

 all the classes with the order beneath them. All such transitory 

 forms, as cystic worms, cercaria, &c., which are mere stages of 

 development from the egg to the perfect animal, are struck out of the 

 general classification and placed among the characteristics of species. 

 The consideration of their embryology will shew that there would be 

 as great impropriety in classifying cercaria, as a larva or chrysalis of 

 insects. Such names as cercaria, redia, &c., will be retained, but 

 with the proviso that they do not indicate species, but different s^asyes 

 of species. 



The general characteristics of the orders infesting man will now be 

 given. In their embryology such additional particulars will be given 

 as are necessary to elucidate the subject. 



Oedeb 1. Cestoidea. 



The body is soft, elongated, flattened, jointed, terminated anteri- 

 orly in a cephalic enlargement to which it is mostly united for some 

 time. The whole together constitutes a strobila. The head or scolex 

 is pyriform in shape, and furnished with four (or two) sucking discs 

 often also supplied with booklets. 



The joints or proglides are destitute of external organs and have 

 embryos armed with hooks. They have no intestine The ces- 

 toidea have no true alimentary canal. In the taenia there are merely 



