114 IXVEKTEBUATE ANIMALS. 



or Round Woiius, comprising the Thorn-headed Worms (Acantko- 

 cephala), the Hair-worms (Qordiacea), and the typical Hound-worms 

 and Thread-worms {Nematoidea). A third division must be con- 

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



SECTION A. PLATYELMIA. 

 OrDEK I. T^NIADA. 



This order comprises the singular animals known as Tapeworms, 

 all of which are found living as parasites within the alimentary 

 canal of other animals ; the young (often called " Cystic woi-ms " or 

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

 solid tissues. As a general 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 species 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 only different individuals, but they belong to 

 different kinds of animals — the two, however, being so far related 

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

 by the " final lK),st." 



Many animals aie infested by ta]jeworras ; but all the leading 

 points of interest in the order will be brought out by a considera- 

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

 subject — namely, the Pork-tapeworm, or Tania solium. The Pork- 

 tapewoi-m is found inhabiting the intestines of man, one only being 

 generally present in the same individual. In shape (fig. 72) it is an 

 extremely elong;ited, flattened, tape-like body, many feet in length, 

 and composed of a number of flattened joints all loosely united to 

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

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

 a point is reached where the organism is iirmly fixed to the mucous 

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

 li, a). The organs by which attachment is effected are, in this 

 species, a crown of recurved hooks and four suckers. In other 

 types only tljc suckers are present (fig. 7.', c). The head is in reality 

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



