118 INVEKTEBEATE ANIMALS. 



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

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

 Tmnia 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 head. The so-called 

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

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

 solid organ of the human body. 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 spoken of as " Echinocooci." In reality, the 

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

 "scolex" of the Timia ccldnococciis of the dog. In this pai'tioular 

 case the " scolex," or young tapeworm, has the power of throwing 

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

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

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

 " scolex " goes on constantly growing and increasing in size, by 

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

 " hydatid tumours " usually give rise to serious or fatal results by 

 the pressure 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 commonly called, are internal parasites, inhabiting vai-ious situa- 

 tions in diff'erent animals, but especially aff'ecting birds and fishes. 

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

 and are furnished with one or more ventral suckers, by which they 

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

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

 which is simply hollowed out of the tissues of the body. No anal 

 opening is present ; and there is no body-cavity. The integument 

 is not ciliated, and the sexes are generally miited. 



The best known of the Trematode worms is the common Liver- 

 fluke {Distoma hepaticmn) which inhabits the gall - bladder and 

 bile-ducts of the sheep and other domestic animals, and sometimes 

 occurs in the same situation in man. In the sheep it gives rise 

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

 Distoma hepcitimnn, or in such a form as Distoma lanceolatiim (fig. 

 74), the body is ovate and leaf -like, half an inch or more in length, 



