BIOLOGY OF THE CESTODES 307 



the bowel, or in the stomach, and may even make their way forward 

 into the oesophagus. 



They also invade other abdominal organs through abnormal com- 

 munications, or through any that may be temporarily open between 

 the intestine and such organs ; they thus reach the abdominal cavity or 

 the urinary bladder, or they work their way through the peritoneum. 



They produce changes in the intestinal mucous membrane at the 

 place of their attachment, the alterations varying in intensity accord- 

 ing to the structure of the fixation organs. The mucous membrane 

 is elevated in knob-like areas by the suckers ; the epithelial cells 

 become atrophied or may be entirely obliterated. Dipylidium 

 caninum bores into the openings of Lieberkiihn's glands with its 

 rostellum, dilating the lumen to two or three times its normal size, 

 while the suckers remain fixed between the basal parts of the cells. 

 Species with powerful armatures penetrate deeper into the submucosa, 

 and some that are not provided with exceptionally strong armatures, 

 or are even unarmed, may be actually found with the scolex embedded 

 in the muscles of the intestinal walls or even protruding beyond 

 (Tcenia tetragona, Mol., in fowls, etc.). Other species, again, even 

 cause perforation of the walls of the intestine of their hosts. 



It is generally assumed that tapeworms, which almost without 

 exception live in the gut of vertebrates, get their nutriment from the 

 gut contents, which apparently they absorb through the whole body 

 surface (cuticular trophopores). In favour of this view is the 

 existence of fat drops in the proglottids, the identity in colour in 

 certain forms between that of the fresh worm and the gut contents 

 and the passage of certain substances derived from medicines (iron 

 and mercury preparation) into the worms in the gut, etc. Whether 

 the suckers are concerned in the absorption of nutriment and to 

 what extent is still questionable. 



THE LENGTH OF LIFE OF THE ADULT TAPEWORM certainly varies ; 

 as a rule it appears to last only about a year ; in other cases (Ligida) 

 it averages only a few days, but we are likewise aware that certain 

 species of Cestodes of man attain an age of several or many years 

 (thirty-five). The natural death of Cestodes often appears to be 

 brought about by alterations in the scolex, such as loss of the hooks, 

 atrophy of the suckers and rostellum, finally the dropping off of the 

 scolex; it is unknown whether a chain of segments deprived of its 

 scolex then perishes or whether it first attains maturity. It has already 

 been mentioned that in a few species the foremost proglottids are 

 transformed into organs of fixation on the normal loss of the scolex. 



Abnormalities and malformations are encountered relatively frequently in the 

 Cestodes such as abnormally short or long segments ; the so-called triangular 

 tapeworms, which if belonging to the TceniidcE always possess six suckers ; often 



