160 INVEHTEBBATE MORPHOLOGY. 



the human digestive tract by fondling, and especially by kissing, infected pet 

 dogs, and the six-hooked embryo makes its way to the liver, lungs, brain, 

 or other organs, where it becomes encysted, producing tumors which, 

 especially in the liver, may reach a great size and a weight of from 10 to, 

 in some cases, 15 kilogrammes. 



Domestic animals are also apt to be infected with Cestodes in addition 

 to those already mentioned, occasionally with fatal results. This is es- 

 pecially the case with sheep, in whose intestine T. expansa may develop in 

 such numbers as to occlude the lumen, and cause death, especially in young 

 iambs. A Coenurus also occurs occasionally encysted in the brain of sheep, 

 producing a disease known from its symptoms as the "staggers," which 

 may likewise result fatally. 



The Relationships of the Cestodes. — In considering the afSnities of the 

 Cestoda, the nature of the strobila, so far as its individuality is concerned, 

 must be inquired into. Two views upon this point are open. The older 

 one regards the Cestode as a colony, considering each proglottid an indi- 

 vidual equivalent to the scolex, and the process of strobilation one of 

 reproduction by budding. Oq this view the strobila is exactly comparable 

 to the Scyphostoma strobila, the scolex corresponding to the Scv-phostoma 

 base and the proglottids to the Ephyrse. There i.s undoubtedly much to be 

 said in favor of such a view which regards the reproduction of the Oestoda 

 as a process of alternation of generations, but at the same time it must be 

 recognized that the buds or proglottids are not reduplications of the parent 

 bud as is the case with Microstoma, where the budding individual has the 

 adult form. In the Scyphostoma strobila the buds do differ from the 

 parent which gives rise to them ; but the Scyphostoma is a larva which 

 gives rise by budding to the adult form, and is comparable rather to the 

 Cysticercus than to the scolex of the Cestode. Non-strobilating Scyphos- 

 tomas become medusae, but the scolex never becomes a proglottid, and 

 the latter cannot be considered the terminal stage of the life-history in the 

 same sense as a medusa is. The nervous system of the entire Cestode 

 strobila centres in the brain of the scolex, the various proglottids never 

 developing independent brains, the reproductive organs being practically 

 the only organs which are reduplicated in successive buds. 



According to the second view the strobila is an individual, and the 

 strobilation is regarded as a culmination of the 'reduplication of organs 

 seen in many forms, but more especially in the Nemerteans (q.v.). This 

 view receives strong support from the occurrence of such forms as Cary- 

 ophyllcBus, Ligula, and Trvjenophorus, described on a preceding page, in 

 which may be seen successive gradations of strobilation, beginning with a 

 simple reduplication of the reproductive apparatus in Ligula, this redupli- 

 cation being accompanied in Trianophoriis by a tendency for the body to 

 constrict into parts, each of which contains one of the sets of reproductive 

 organs. 



The choice between these two views hinges upon the question of indi- 

 viduality. The individuality of either Ligula or Tricenophorns can hardly 



