PL A TUELMINTHES. 



267 



PHYLUM IV. PLATHELMINTHES (FLATWORMS). 



This group is well characterized by the name. AVith few 

 exceptions (rhabdocceles, many trematodes) the nearly flat ventral 

 surface and the slightly arched back are closely approximate and 

 pass with a more or less sharp margin into each other. In many 

 cases the ventral surface is distinguished by its lighter color. In 

 all the bilaterally symmetrical body is composed of a solid paren- 

 chyma, a mass of connective tissue traversed by muscle fibres, in 

 which the various organs alimentary tract, nerves, excretory and 

 reproductive organs are imbedded. In the lower forms the di- 

 gestive system is markedly like that of the co3lenterates ( Actinozoa, 

 Ctenophora) in that there is but a single opening and this leads by 

 an ectodermal oesophagus (stomodaeum) to the interior. In para- 

 sites the digestive tract may be lost. The skin is a one-layered 

 epithelium, sometimes ciliated, sometimes protected by a thick 

 cuticula. Inside this comes a muscular layer (fig. 225) in which 



FIG. 225. Transverse section fright half) of a Planarian. ri, vitellaria; dv, dorso- 

 ventral muscle fibres; e, ectodermal epithelium with cilia; {/, gastric diverticula; 

 7), testicular follicles ; lm, longitudinal muscles (dots, in section) ; n, lateral nerve 

 cord. 



longitudinal muscles are always present, and in addition frequently 

 circular and oblique muscles, as well as those passing from dorsal 

 to ventral surfaces. The nervous system (fig. 228) consists of a 

 pair of ganglia (' brain') in front of (i.e., above) the oesophagus 

 and longitudinal nerves leading backwards from it. The excretory 

 organs (fig. 226) are composed of a series of tubes, the protone- 

 phridia or water- vascular system/ which branch and ramify the 

 parenchyma. In most the sexes are united in one individual and 

 the reproductive organs take up considerable space. There is a 

 small paired or unpaired ovary and vitellaria, usually paired and 

 branched. The eggs arise in the ovary, and to these are added 

 nourishment in the shape of cells (abortive ova) rich in yolk from 



