ORDERS OF TREMATODA 223 



Order 2. Aspidocotylea, with a large sucker occupying most of 

 the ventral surface. Development is direct, and there is one host. 

 e.g. Aspidogasier in Molluscs. 

 Order 3. Malacotylea, with never more than two suckers. The 

 development is indirect and requires two hosts, the adult usually 

 frequenting the gut of a vertebrate. 



e.g. Disiomum, with numerous species. Schistosomum (Bilharzia) 



hcematohium, a parasite of man, widely distributed in Africa, 



e.g. in Egypt. It occurs in the portal vein, the blood vessels 



of the bladder, etc., 'causing inflammation, ha;maturia, 



stone, etc. The embryos are passed out in the urine. 



The intermediate host is a fresh-water snail {e.g. Biilinus). 



There is no redia. The bifid microscopic Cercaria usually 



enters man by the skin. The pain is due to the sharp 



corners of the egg-shells, which have terminal spines. 



Another species, S. mansoni, is intestinal, and the eggs, 



which have a lateral spine, pass out with the faeces. The 



young stages occur in Planorbis, etc. The cercariae die 



in 36 hours in water kept quite still, or may be killed 



by a little sulphate of soda. 



Monostomum, with one sucker ; adult in ducks, young in 



fresh-water snail, Planorbis. 



The relationships of the Trematodes are on 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) and a tape-worm (strobila) 

 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. Taenia 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 imperfectly integrated? The general form of 

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

 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- 



