TYPE PLATYBELMINTHE8. 



127 



CHAPTER VII. 



TYPE PLATYHELMINTHES. 



The Platyhelminths constitute a group whicli, though 

 presenting a much higher grade of organization than the 

 Coelentera, nevertheless show certain general structural 

 similarities to the representatives of that type. Thus upon 

 the exterior of the body there is a thin ectoderm (Fig. 69, 



bm vd ■» 



Fig. 69. — DrAGRAMMATic Transveesb Sections through yahious Tuebel- 

 LAEiA. A, an Acoelan ; B, an AUoioctelan ; C, a Rbabdoccelan ; D, a 



Triclad. 



bm = basemeut-membrane. 

 d = iutestiue. 

 ec = ectoderm. 

 m = muscle-layer. 

 n = nerve. 



o = ovary. 

 od = oviduct. 

 p = pareuchyme. 

 t = testis, 

 V = vitellarium. 



vd = vas deferens. 



D, ec), below which is a basement-memhrane {bm) sometimes 

 thin, structureless, and destitute of cells, sometimes thicker 

 and enclosing branched cells, and strictly comparable to the 

 mesoglcea of the Coelenterates. Within the basement-mem- 

 brane there is a compact mass of tissue surrounding, in the 



