lis INVEliTEBKATE ANIMALS. 



the sheep, in which animal it produces the disease known as the 

 "staggers." Lastly, another curious tapeworm of the dog (viz., the 

 Tcenia echinococcus) lives in its young stages in the human body, 

 giving rise to what are known as " hydatid tumours." The adult 

 tapeworm of this species is only about a quarter of an inch in length, 

 and consists only of three joints besides the liead. Tlie so-called 

 " liydatid tumour " consists of one or more bladder-like tumours, 

 varying greatly in size, endjedded in the liver, lungs, brain, or other 

 solid organ of the human liody. The bladder is filled with a watery 

 fluid, and its inner surface carries a vast number of little buds, 

 which are mostly similar in structure to an ordinary " cystic worm." 

 Similar little "cystic worms" are often found floating in the fluid 

 itself, and are commonly s))oken of as " EMnococci" In reality, the 

 " hydatid tumour " is the result of the development of the young or 

 " scolex " of the Tania eehini>eoi-cus of tlie dog. In this particular 

 case the "scolex," or young tapew(jrm, has the power of throwing 

 out buds, and thus of forming a comjiosite organism or colony. 

 Hence, in this case, instead of remaining harmlessly embedded in 

 tlie tissues, as is usual among the young of the tapeworms, the 

 " scolex " goes on constantl}^ growing and increasing in size, by 

 the production of new buds in its interior. For this reason, also, 

 " hyd.itid tumours" usually give rise to serious or fatal resvilts by 

 the jjressure which they exert upon the tissues or organs amongst 

 which they lie. 



Order II. Trematoda. 



The " suctorial " worms, or "flukes," as the members of this order 

 are connnoidy called, are internal parasites, iidiabiting various situa- 

 ticjns in difl'erent animals, but especially affecting bii-ds and fishes. 

 They are almost all more or less flattened and leaf-like in shape, 

 and are furnished witli one or more ventral suckei's, Viy which they 

 adhere. They are distinguished from the Tioiiada by always pos- 

 sessing an alimentary canal, which is often miich branched, and 

 which is simply hollowed out of the ti.ssues of the body. No anal 

 o])ening is j)re.sejit ; and there is no body-cavity. The integument 

 is not ciliated, anil the sexes are generally united. 



The liest kncjwn of the Trcmatode worms is the common Liver- 

 Huke {DiKtomit lirpatifinn) which inhabits the g.ill - bladder and 

 liile-ducts of the sheep anil other domestic animals, and sometimes 

 oi'i-ius in the same situation in man. In the sheeji it gives rise 

 to till' prevalent and often fatal disease known as the "rot." In 

 Dixtniiiii lii'jiiitlfiiiii, or in such a form as DUtoma taiiceolatum (fig. 

 74), tlie body is ov;ite and leaf-like, half an inch or more in length. 



