PLATIIELMINTEE8. 267 



PHYLUM TV. PLATIIELMINTHES (FLATWORMS). 



This group is well characterized by the name. AVith few- 

 exceptions (rhabdocogles, many trematodes) the nearly flat ventral 

 surface and the slightly arched hack 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 \)j 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 cadenterates ( Actinozoa, 

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

 an ectodermal 03SOf)hagus (stomoilajum) 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 (rittht half) of a Planarian. it, vitellaria : (fr, ilorso- 

 vi'ntral muscle fibres ; e, ectodermal epithelium Willi cilia ; i/, u'a^rrii' il i\-crTirii In ; 

 /). testicular follicles ; (iii, loiigitmlinal muscles (dots, in secliijiii ; », lateral aci've 

 eoril. 



longitudinal muscles are always present, and inadditiim frequently 

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

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

 pair of ganglia ('brain ■) in front of (/.c, above) the resopdiagus 

 and longitudinal nerves leading backwards frtmi it. The excretory 

 oro-ans (fig. 22G) are composed of a series of tubes, the protone- 

 ]>hridia 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 viielldria, usually paired and 

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

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



