PHYLUM PLATYHELMINTHES 



249 



)mogeneous, outer cuticle; a cellular, or at least, nucleated, 

 epidermis, and a basement membrane ; but the cellullar epidermal 

 layer is absent as such in the adult condition in the majority of 

 the Trematodes, and there is only a homogeneous, non-nucleated 

 outer layer, which may be the modified epidermis, or may be the 

 cuticle, with or without a basement-membrane. Rhdbdite-forming, 

 and other unicellular glands derived from the epidermis, are 

 frequently present beneath the integument. 



In the Cestodes, as in the majority of the Trematodes, no 

 definite epidermis is present. The external layer, sometimes 

 divided into two, is of a homogeneous non-cellular character, and 

 is usually termed cuticle, though it perhaps partly corresponds to 

 the basement membrane of other groups. Beneath this is a thin 

 layer which appears to consist of elastic fibres. Beneath this again 

 is a layer of fusiform cells, narrow prolongations of which pass to 

 the surface. It is possible these may be concerned in the absorp- 

 tion of nutrient matter, but some of them are undoubtedly of the 

 nature of nerve-cells and have nerve-fibres connected with them. 



The muscular layers of the body-wall vary somewhat in their 

 arrangement in the different groups of Platyhelminthes. Most 

 commonly there is an external layer of circularly arranged, and 

 an internal layer of longitudinally arranged fibres ; frequently 

 layers of fibres running in a diagonal direction are present also. 



Characteristic of the flat-worms is a peculiar form of connective 

 tissue, the parenchyma (Fig. 199), mention of which has^already 



ct, 



FIG. 199. Parenchyma, a, I, intercellular spaces ; bm. basement membrane ; <', nuclei ; c?, nuclei ; 

 ep. epidermis. (After -Braun.) 



been made in the descriptions of the examples, presenting many 

 varieties, filling up the interstices between the organs leaving 



