PLATHELMINTHES 



239 



is composed of a solid parenchyma, a mass of connective tissue traversed 

 by muscle fibres, in which the various organs alimentary tract, nerves, 

 excretory and reproductive organs are imbedded. The digestive 



FIG. 212. Transverse section (right half) of a Planarian. d, vitellaria; dv, dorso- 

 ventral muscle fibres; e, ectodermal epithelium with cilia; g, gastric diverticula; h, 

 testicular follicles; lin, longitudinal muscles (dots, in section); n, lateral nerve cord. 



system has various shapes; in the lower groups it resembles that of the 

 ccelenterates in that there is but a single opening and this leads by an 

 ectodermal oesophagus (stomodaeum) to the interior (fig. 59). In 



parasites the digestive tract may be lost. The 

 skin is a one-layered epithelium, sometimes cili- 

 ated, sometimes protected by a thick cuticula. 

 Inside this comes a muscular layer (fig. 212) in 

 which longitudinal muscles are always present, 

 and in addition frequently circular and oblique 

 muscles, as well as those passing from dorsal to 



FIG. 213. FIG. 214. 



FIG. 213. Excretory system of Cercaria (after Albert Lang), b, limb of bladder; 

 b', same with urinary concretions; cc, collecting canal; cs, canals of ventral sucki-r; cr, 

 collecting vacuole; e, eye; ep, excretory pore; /, lumen of tail; os, oral sucker; vs, ventral 

 sucker. 



FIG. 214. Eggs of Distomum nodulosum (after Schauinsland). A, before de- 

 velopment; B, later, the yolk broken down. </, yolk cells; ei, egg cell; ek, ectoderm; 

 en, entoderm; p, pigment spot. 



ventral surfaces. The nervous system (fig. 215) 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. 213) are 



