TYPE PLATYUELMINTHES. 



127 



CHAPTER VII. 



TYPE PLATYHELMINTHES. 



THE Platyhelmiuths constitute a group which, 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, 



vd 



ec 



tm 



TL 



bin vd -a. 



FIG. 69. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH VARIOUS TURBEL- 

 LAP.IA. A, au Accelan ; B, au Alloiocoelan ; C, a Rhabdoccelau ; D, a 

 Triclad. 



bm basement-membrane. o = ovary. 



d intestine. od = oviduct. 



ec ectoderm. p = pareuchyme. 



in = muscle-layer. t = testis 



n = nerve. = vitellarium. 



vd vas defereus. 



D, ec), below which is a basement -membrane (bm) sometimes 

 thin, structureless, and destitute of cells, sometimes thicker 

 and enclosing branched cells, and strictly comparable to the 

 mesoglo3a of the Ccelenterates. Within the basement-mem- 

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



