134 ANIMAL PARASITES. 



means of which they attach themselves. There are several genera and 

 species found in man. The most common genus is Distoma. Of these 

 I), hepaticum is of most frequent occurrence (Fig. 57). It is about 30 

 mm. long, and usually occurs in the gall ducts and gall bladder. The 

 embryos are often attached to water plants, from eating which the 

 infection is believed to occur. D. lanceolatum is more slender, pointed 

 at the ends, 8 to 10 mm. long, and has been 

 found a few times in the gall bladder D. sinense 

 is a slender worm about 15 mm. long, and has 

 been found in the bile in considerable num- 

 bers, particularly in the Chinese. D. hawiato- FIG - 57 ;^ D u ' t ra l ] I ICUM ' 

 Mum is a more nearly cylindrical worm; the 



sexes are distinct ; the male from 12 to 14 mm. long, the female 16 to 

 18 mm. long, and occurs, especially in Egyptians, in the portal and 

 other abdominal veins. 



CESTODA (Tape Worms). 



These important worms consist, in the mature state, of more or less 

 rectangular or elongated segments, each one of which represents a single 

 individual, arranged in a linear series to form a colony, at one end of 

 which, called the head, is a variously formed structure for the attach- 

 ment of the colony to its host. The neck and 



3pp^)X^,?~^P head are called the scolex, while the segments 



are called proglottides. These worms have 

 neither mouth nor alimentary canal. They 

 are hermaphrodites, the sexes being united in 

 *|4 the proglottides. The head and neck (scolex) 

 \ U may exist as an immature form in various tis- 

 sues and organs where they are encysted, and 

 V. ;^ are often called cysticercus. 



Tcenia solium is of frequent occurrence in 



\. man. It may be several metres in length, and 



may be coiled up or stretched out in the small 



'(. ,.". './'/ intestines. Several worms may be in the gut 



at one time. The head, about the size of a 

 pin's head (Fig. 58), has a projecting probos 

 cis or rostellum, around which are arranged 



, ' ' , a double row of horny booklets. Below these 



; ;;.-;.- are four sucking discs at the sides of the head. 



The booklets of the anterior row are larger 



than those in the posterior row, and are from 



FIG. W. HEAD OF T^NIA SOLIUM, 



x about 40. 0.16 to 0.18 mm. long. The proglottides, 



when fully developed, are from 10 to 12 mm. 



long and from 5 to 6 mm. wide, but those nearest the head are much 

 shorter and immature. The eggs of T. solium are ovoidal structures, 

 about 0.03 mm. in diameter. The embryo of this worm is most com- 



