156 PLATHELMINTHES 



SuBPHYLUM 1. PLATHELMINTHES. 



Flat worms. Flattened or in some cases cylindrical worms of soft 

 texture which are found in the water or in moist earth, or which live as 

 parasites in animals or plants. The body is without a distinct head or 

 paired appendages and is not metamerically segmented. A body cavity is 

 also wanting in most of them, the spaces between the internal organs being 

 secondarily filled with a vesicular connective tissue, called parenchyma. 

 The outer surface of the body is either a ciliated epithelium or a thick 

 unciliated cuticula and no hard skeletal structures are present except 

 chitinous hooks and sj^jines. The mouth is usually in the ventral surface 

 in the Turbellaria and at the front end of the body in the other groups, 

 and an anus is not present, except in the Nemertea. A mouth and an 

 alimentary tract are wanting in the tapeworms. The nervous system 

 consists of paired cerebral ganglia forming a brain at the fonvard end 

 and nerves extending to various parts of the body. Special sense organs, 

 when present, consist of simple eyes, tentacles, or statocysts. The excretory 

 system consists of slender tubes extending throughout the parenchyma, 

 the final branches of which end in flame cells. It opens to the outside 

 either through a single pore or through several paired pores. No special 

 respiratory organs are present, and except in the Nemertea, no circulatory 

 organs or blood fluid. The reproductive organs are complex, except 

 among the Nemertea, hermaphroditism being general. Asexual reproduc- 

 tion by budding or fission is common in certain groups. 



History. — Certain of the parasitic flatworms have been knoAvn from 

 time immemorial. Linnaeus included all invertebrates except arthropods 

 (his Insecta) in the class Vermes, one of the orders of which was the 

 Intestina, or worms proper. Cuvier (1798) first called attention to the 

 fundamental distinction between the unsegmented and the segmented 

 worms, to the former of which Rudolphi (1808) gave the name Entozoa, 

 most of the unsegmented worms as then known being parasites. It was 

 this author who, following however Zeder in his general classifications, 

 laid the foundation of our present classification of parasitic worms, of 

 which he formed five orders, the roundworms or Nematodes, the Acan- 

 thocepJiala, the Trematodes, the tapeworms or Cestodes, and the bladder- 

 worms or Cystici. F. S. Leuckart and von Baer showed that these groups 

 did not necessarily bear a genetic relationship to one another. Vogt in 

 1851 first joined the four orders of flatworms to form a class which he 



Wnodworth, 1895. "Flatworms and Mesozoa, Nemertines, Thread-Worms and Sa- 

 gitta, Rotifers," etc., Cambridge Natural History, Vol. 2, 1806. "Les Ver- 

 midiens," by Delage et H6rouard, Traits de Zool. Concrete, Vol. 5, 1897. "A 

 Student's Textbook of Zoology," Vol. 1, by Adam Sedgwick, 1898. "A Treatise on 

 Zoology, Part 4," edited by E. Ray Lankester, 1901. 



