190 UNSEGMENTED “WORMS,” 
but becomes a parasite on the gills of a minnow, and 
there two individuals unite very closely and permanently. 
Tristomum, with three suckers, on some marine fishes. 
Order 2. Aspidocotylea, with a large sucker occupying most 
of the ventral surface. Development is direct, and there is one host. 
e.g. Aspidogaster 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. Distomum, with numerous species. Sch¢stosomum ( Belharzia) 
Aematobium, a parasite of man, widely distributed in Africa, 
e.g. in Egypt. It occurs in the portal vein, the blood vessels 
of the bladder, large intestine, etc., causing inflammation, 
heematuria, stone, 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 gynze- 
cophoric canal. Man is probably infected through impure 
water, but the intermediate host is still unknown. The 
embryos are passed out in the urine. 
Monostomum, with one sucker; adult in ducks, young in 
fresh-water snail, 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 CEstopa. Tape-worms 
The Cestodes ave 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 ts unsegmented, e.g. Archigetes and Caryophylleus, 
with one set of gonads ; in a few others, e.g. Ligula, there 
ts a serial repetition of gonads without distinct segmentation 
of the body ; in most cases, e.g. Tenia and Bothriocephalus, 
the body of the tape-worm forms a chain of numerous joints or 
proglottides, cach 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 ts 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 ts parenchymatous connective 
tissue, often with cortical deposits of lime, and at least two sets 
(longitudinal and transverse) of unstriped muscles. The 
