264 



ZOOLOGY 



b.m 



elm 



i.l.m. 



stinging capsules occur similar to those of the Ccelenterata, and 

 transition-forms between rhabdites and stinging capsules occur in 



some cases. Adhesive cells with 

 rh processes also frequently occur 



in the epidermis. Beneath the 

 epidermis is a basement mem- 

 brane (b. m.), which in the 

 Polycladida is of a thick re- 

 sistent character, and contains 

 stellate cells. 



In a small number of the 

 Trematoda three layers are 

 distinguishable in the integu- 

 ment — a homogeneous, or 

 nearly homogeneous, outer 

 cuticle ; a cellular, or at least, 

 nucleated, epidermis, and a 

 basement membrane; but the 

 cellular 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. 

 Rhabdite-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 or more strata, is of a homogeneous non-cellular 

 character, and is usually termed cuticle. Beneath this is a thin 

 layer of parenchyma, the basal membrane. Beneath this again 

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

 to the cuticle, into the inner part of which they penetrate and 

 spread out into a thin layer. These cells are by some authors 

 regarded as the cells that secrete the cuticle ; but they may be 

 concerned in the absorption of nutrient matter, and 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. 210) — mention of which has already 



d.v.rn. 



Fig. 209.— Section of the body-wall of a Triclad 

 6. m. basement membrane ; c. in. circular 

 muscles ; d. v. m. dorso-ventral muscles ; 

 e. 1. ni. external longitudinal muscles ; ep. 

 epidermis ; i. I. m. internal longitudinal 

 muscles ; p. parenchyma ; rh. rhabdites ; 

 rh. c. rhabdite-forming cells. (After Jijima.) 



