PHYLUM PLATYHELMINTHES1 



237 



od 



FIG. 188. Transverse section of aJPlanarian. circ. mus. circular muscular fibres ; ccec. in 

 testinal caeca ; dors. vent. mus. dorso-ventral muscular fibres ; eel. ectoderm ; ext . long. mus. 

 external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. 

 long. mus. internal layer of longitudinal muscular fibres ; ne. nerve-cords ; o. d. oviducts ; 

 par. parenchyma ; test, testes ; v. def. vas deferens ; vit. vitelline glands. (From 

 Hatschek's Lehrbuch.) 



(Fig. 189), with a triangular process, the head-lobe, projecting 



from the broader end. The symmetry of the parts is distinctly 



bilateral, as in the Planarian. Externally 



the body is quite equilateral, the right and 



left portions exactly balancing one another, 



but, as will appear subsequently, this complete 



symmetry does not extend to all the internal 



organs. 



The surface is devoid of vibratile cilia, but 

 is covered with innumerable minute spinules 

 or papillce, which are prolongations of the 

 homogeneous external layer or cuticle investing 

 the whole animal. At the extreme anterior 

 end of the triangular head-lobe is the small ria 18 9._ r a s c i o i a 

 opening of the mouth (mo.) surrounded by a 

 muscular oral sucker. A short distance back mouth; repr. 

 on the ventral surface, just behind the head- Sor^sucS. ; sckr ' pos 

 lobe, is a second much larger posterior sucker 

 (sckr.). Between the two suckers, but rather nearer the pbsterior 

 one, is a median aperture, thegenitctl opening (repr.), through which a 

 curved muscular process, the cirrus or penis, may be protruded. In 

 the middle of the posterior end of the body is a minute opening, 

 the excretory pore (excr.). 



Body-wall. The body-wall (Fig. 190) is found on section to 

 comprise three layers : (1) a homogeneous cuticle (cut.) of which 

 the spinules (sp.) are special developments ; (2) a layer of circularly 

 disposed muscular fibres (circ. mus.) : (3) a layer of longitudinal 

 muscular fibres (long. mus.). A cellular epidermis is wanting. 

 Beneath the muscles are numerous unicellular glands (gl.), the 

 ducts of which, in the form of processes of the cells, open on the 



