TURBELLARIA. 



211 



nucleated material, which bears cilia and rests upon a stratified 

 basement membrane (Fig. 166). It is covered externally by a 

 homogeneous membrane bearing tactile hairs and comparable to 

 a cuticle. Peculiar rod-like structures, called rhabdites^ are found 

 in the ectoderm cells and in cells lying deeper in the parenchyma, 

 but connected with the ectoderm layer by processes. The rhabdites 

 are homogeneous, highly refractile structures of unknown function, 

 and can be extruded from the ectoderm. 

 Other structures probably of a similar $ 



character, but differing slightly, are also 

 found in the ectoderm these are the 

 pseudorJudxlites and sayittocysts. Nema- 

 tocysts are found in some members of 

 the class (Microstoma, Anonymus, Stylo- 

 choplana). Various pigments are often 

 present in the epidermis or in the 

 parenchyma; and green vesicles con- 

 taining chlorophyll and starch grains 

 (Vortex), or yellow cells (Convolute), 

 are found in the parenchyma in some 

 forms C? symbiotic Algae). The dermis, 

 or outer layer of the parenchyma, lies 

 beneath the basement membrane, and 

 contains well-developed muscular layers 

 (usually outer circular and inner longi- 

 tudinal). 



The structure of the parenchyma is 

 difficult to make out. In the Acoela 

 the whole of the tissues within the 

 basement membrane may be described 

 as consisting of a plasmatic mass con- 

 taining nuclei, vacuoles, and fibres, 

 both muscular and connective. The 



centre of this mass is of a softer consistency than the outer 

 parts, and constitutes the solid digestive endoderm into which 

 the food passes to be digested. The outer part of the paren- 

 chyma, which is not at all marked off from the central, contains 

 dermal muscles externally, networks of connective tissue fibres, and 

 muscular fibres running dorso-ventrally through the body. Further, 

 the central nervous mass and the nerves and the gonads are all parts 

 of this parenchyma, differentiated from it indeed, but not in any 



FIG. 166. Portion of a longitudinal 

 section of Planaria polychroa, 

 showing the ectoderm and outer 

 part of the parenchyma. R rhab- 

 dites in ectoderm ; K~L nuclei of 

 ectoderm ; Bm basement mem- 

 brane ; Pg pigmented connective 

 tissue cell ; K 2 nucleus of paren- 

 chyma cell ; Rz deep rhabdite- 

 forming cell (after Leuckarfc and 

 Nitsche). 



