114 IXVEKTEBUATE ANIiMALS. 



or Round Worms, comprising the Thorn-headed Worms (Acantfio- 

 cephala), the Hair-worms (Gordiaceci), and the t3^pical Round-worms 

 and Thread-worms (iVematoidea). A third division must be con- 

 stituted for the reception of the Wheel-animalcules {Rotifera). 



SECTION A. rLATYEI.MIA. 

 Order I. T.eniada. 



This order comprises the singular animals known as Tapeworms, 

 all of which are found living as ])arasites within the alimentary 

 canal of other animals ; the young (often called " Cystic worms " or 

 " Bladder-worms ") being also parasitic, liut usually inhabiting the 

 solid tissues. As a geneial rule, the adult tapeworm inhabits the 

 alimentary canal of an individual of some one species of the higher 

 animals ; while the young of the same tapeworm inhabits the tissues 

 of an individual belonging to some other sjiecies of animal. As a 

 rule, therefore, each tapeworm is, at different times of its life, 

 parasitically related to two different animals, belonging generally 

 to different species. In its early stages each tapeworm inhabits 

 the tissues of what may be called its " intermediate bearer," while 

 in its adult condition it inhabits the intestine of what we may term 

 its " final host." As a rule, the " intermediate bearer " and the 

 " final host " are not oxdy ditferent individuals, but they belong to 

 difl'erent kinds of animals — the two, however, beiiig so far related 

 to each other that the "intermediate bearer" is liable to be eaten 

 by the "final host." 



Many animals are infested liy tapewoi'ms ; but all the leading 

 points of interest in the oi'der will lie brought out by a considera- 

 tion of one of the commonest of the tapeworms to which man is 

 subject — namely, tlie Pork-tapeworm, or Tieiiia solium. The Pork- 

 tapeworm is foiinil inhabiting the intestines of man, one ordy being 

 generally pi'esent in the same individual. In shape (fig. 72) it is an 

 extremely elongated, flattened, tajie-like body, many feet in length, 

 and comjjosed of a number of flattened joints all loosely luiited to 

 one another. At one extremity the joints, technically called the 

 "proglottides," liecome much smaller and narrower, till ultimately 

 a point is reacheil where the organism is firmly fixed to the mucous 

 membrane of the intestine by means of a minute nunided head (fig. 

 li, rt). 'J'he organs by which attachment is eM'ected aie, in this 

 K|ic(:ics, a crown of recurved hooks and four suckers. In other 

 types (jiily the suckers arc ]iresent (fig. 73, c). The head is in reality 

 the tiiie animal, and all the long, jointed, tape-like body which 



