212 THE ANIMAL PARASITES OF MAN 



CLASSIFICATION OF THE PLATYHELMINTHES. 



Class /. Turbellaria (or Eddy Worms). Flat worms for the most part, free 

 living, and always covered with a ciliated epithelium. 



Order i. Rhabdoccelida, gut unbranched. 



Order 2. Tridadida, gut with three main branches. 



Order ^.Polydadida, a central gut with lateral caeca. Development direct 

 or through metamorphosis. They live in fresh and salt water or on land ; 

 very seldom as parasites. 



Class II. Trematoda (Sucking Worms 1 ). [Usually known as Flukes. F. V. T.] 

 Flat worms, living as ecto- or endoparasites, that are only ciliated in the larval 

 condition, and in their adult state are covered with a cuticle, the matrix cells of 

 which lie in the parenchyma. They have either one, a few, or several suckers,'' 

 and frequently also possess chitinous fixation and adhesive organs. The intestine 

 is single, but generally bifurcated, and not uncommonly there are transverse anasto- 

 moses between the forks or diverticula on them. Excretory organs double, with 

 two orifices at the anterior extremity or a single one at the posterior end. 

 Development takes place by a metamorphosis or alternation of generations (p. 283). 

 These worms are almost always hermaphroditic, with two or more female and one 

 male sexual orifice. They live, almost without exception, as parasites on vertebrate 

 animals, but the intermediate generations are passed in molluscs. 



Class III. Cestoda (Tapeworms). Endoparasitic flat worms without an alimen- 

 tary canal. The larval stages are rarely ciliated, but are usually provided with six 

 spines ; the adult worm is covered with a cuticle, the matrix cells of which are em- 

 bedded in the parenchyma. The body consists of a single segment (Cestodaria) or a 

 chain of segments, in which case it consists of the scolex and the segments contain- 

 ing the sexual organs (proglottides) (Cestodes s. sir.). The scolex is provided with 

 various adhesive and fixation organs, and there are calcareous corpuscles in the 

 parenchyma. Excretory organs symmetrical, opening at the posterior end. These 

 worms are always hermaphroditic, and then possess one or two female and one 

 male sexual orifice. During development a larval intermediate stage ("measle") 

 occurs and almost always in a different host to that in which the adult sexual worm 

 lives. The adult stage is parasitic in vertebrate animals, but the larval stage may 

 occur in invertebrates. 



Class II. TREMATODA, Rud. 



These worms are usually leaf- or tongue-shaped, but also barrel- 

 shaped or conical; they vary from o'i mm. to almost i m. 3 in 

 length ; most of them, however, are small (5 mm. to 15 mm.). The sur- 

 face on which the orifice of the uterus and the male sexual opening 

 are situated is termed the ventral surface ; the oral aperture, which 

 also acts as anus, is always at the anterior end in the sub-order 

 Prostomata (p. 230), but in the sub-order Gasterostomata it is ventral. 



1 This grouping goes back to the year 1800, and was made by J. G. II. Zeder, a physician 

 and helminthologist of Forchheim, who divided the helminths, which until 1851 were generally 

 regarded as a special class of animals, into the groups of round, hook, sucker, tape and 

 bladder worms, as which they are recognized up to the present time. In 1809, K. A. Rudolphi 

 gave them the names Nematodes, Acanthocephali, Trematodes^ Cestodes and Cystici. 



2 A sucker or acetabulum (little cup) is a round, cup-shaped muscular organ, the muscles of 

 which are sharply defined from those of the body. 



3 Nematobothrium filarina, van Bened., on the branchial chamber of the Tunny. 



