CESTODA. 179 



e.g. Distoinnm, with numerous species. 



Bilharzia hccinatobiiis, a dangerous parasite of man, widely 

 distributed in Africa, e.g. in Egypt. It infests the urinary 

 and abdominal blood vessels, causing inflammation, 

 haematuria, etc. The sexes are separate, and the male 

 (about half an inch in length) carries the more thread-like 

 female (about an inch in length) inserted in a groove or 

 gynrecophoric canal. Man is probably infected from small 

 Crustaceans in drinking water. The embryos are passed out 

 in the urine. 

 JMonostomuni, with one sucker, adults in ducks, young in 



Planorbis. 



The relationships of the Trematodes are on the one hand with the 

 free-living Turbellarians, on the other hand with the parasitic Cestodes. 



Class CESTODA. Tape-worms. 



The Cestodes are internal parasites, whose life history 

 includes a bladder-worm (proscolex) an da tape-worm (strobild) 

 stage, the former in a Vertebrate or Invertebrate host, the 

 latter (with one exception] in a Vertebrate. In a few cases 

 the body is unsegmented, e.g. Archigetes and Caryophyllaeus, 

 with one set of gonads ; in a few others, e.g. Ligula, there 

 is a serial repetition of gonads without distinct segmentation 

 of the body; in most cases, e.g. T?enia and Bothriocephalus, 

 the body of the tape-worm forms a chain of numerous joints or 

 proglottides, each with a set of gonads. Thus the class in- 

 cludes transitions from unsegmented to segmented forms, but 

 the latter are imperfectlv integrated. The general form of 

 the body is tape-like and bilaterally symmetrical, with hooks, 

 grooves, or suckers ensuring attachment to the gut of the host. 

 The body wall consists of a cuticle and a well innervated 

 epidermis, within which there is parenchymatous connective 

 tissue, often with cortical deposits of lime, and at least two sets 

 (longitudinal and transverse] of unstriped muscles. The 

 nervous system consists of two or more longitudinal nerve- 

 strands and anterior commissures there are no special sense 

 organs. There is no alimentary system ; the parasite 

 floating in the digested food of its host absorbs soluble 

 material by its general surface. There is no vascular or 

 respiratory system, and a body cavity is represented merely by 

 irregular spaces in the solid parenchymatous tissue. In some 

 of these spaces there are "flame cells," which lie at the ends of 



