128 THE ANIMAL PARASITES OF MAN 



of the body, consists of cerebral ganglia, united together in the shape of 

 dumb-bells, and of two or several MEDULLARY FASCICULI forming often 

 anastomoses. Organs of sense usually occur only in the free-living species,, 

 more rarely during the free-living stages of a few parasitic species and in a 

 few ectoparasites. 



[In Platyhelminthes simple eye-spots frequently occur, and in a few 

 an auditory vesicle. 



BLOOD-VESSELS and definite RESPIRATORY ORGANS are lacking [except in 

 N ' emertinea] ; the EXCRETORY APPARATUS (formerly also termed water-vascular 

 system) is typical of the entire class. It commences in the interstices of the 

 parenchyma, with peculiar ciliated infundibuli which will be described 

 further, the capillary processes of which go on uniting into larger branches, 

 and finally form two large collecting vessels, which, sometimes separately and 

 sometimes united, open to the exterior through one, two, or numerous pores. 



Nearly all the Platyhelminthes are HERMAPHRODITIC [the exceptions are the 

 Bilharzia, Micvostomidce and Nemertinea F. V. T.], and in nearly all there 

 are in addition to the ovaries producing ova, other glands attached to the 

 female genital apparatus, namely, the yolk glands, which provide a sub- 

 stance termed yolk, which serves as nourishment for the embryo. The 

 fully-formed eggs have shells and are composed of the ovum, which is 

 surrounded by numerous yolk cells or their product of disintegration. The 

 two sexual openings usually lie close together, frequently in the base of a 

 genital atrium ; they are rarely separated from one another. Shell-glands 

 also usually occur. 



Reproduction is sexual, often, however, combined with asexual methods 

 of propagation (segmentation, budding). The Platyhelminthes live partly free 

 in fresh or salt water, exceptionally, also, on land. The greater part, 

 however, live as parasites on or in animals. 



CLASSIFICATION OF THE PLATYHELMINTHES. 



Class I. Turbellaria, or Eddy worms. Flat worms for the most part, 

 free-living and always covered with a ciliated epithelium. [They are 

 usually leaf-shaped or oval. F. V. T.] Their intestine is either simple 

 (JRhabdoccBlida) or ramified (Dendroccelidd}, 1 or they have an entodermal syn- 

 cytium gradually dispersed throughout the parenchyma {Acceld}? Develop- 

 ment direct or through metamorphosis. They live in fresh and salt water 

 or on land ; very seldom as parasites. 



Class II. Trematoda, Sucking worms. 3 [Usually known as Flukes. 

 F. V. T.] Flat worms, living as ecto- or endoparasites, that are only 



1 [These are also known as Polycladida and Tricladida. F. V. T.] 



2 [These are a division of the Rhabdocalida. F. V. T.] 



3 This grouping goes back to the year 1800, and was made by J. G. H. Zeder, 

 a doctor and helminthologist of Forchheim, who divided the helminths, which until 

 1851 were generally regarded as particular classes of animals, into the groups of 

 hook, round, sucker, tape and cyst worms, as which they are recognised up to the 

 present time. - In 1809, however, K. A. Rudolphi gave them the names, Nematodes, 

 Acanthocephali, Trematodes, Cestodes and Cystici. 



