DISTOMUM HEPATICUM 7 



epithelical and muscular tissue in which the following layers 

 may be distinguished:— (i) Externally a so-called cuticle, 

 apparently of homogeneous texture, but exhibiting nuclei at 

 intervals. The presence of nuclei shows that this external 

 layer is a true epidermis formed of metamorphosed cells and 

 not a cuticle, which would be a product of the secretory 

 activity of an underlying layer of cells. This external layer, 

 which may be called the ectoderm, swells up in water and con- 

 tains the chitinoid scales which have already been mentioned. 

 Each scale is a somewhat broad flattened plate rounded at its 

 distal, but truncated at its proximal, extremity. The scales 

 appear to project from the surface, but they are really con- 

 tained in as many pocket-like cavities of the metamorphosed 

 ectoderm, and their projecting ends are covered over with a 

 thin layer of the peculiar ectodermic tissue. They vary in 

 size from '036 to '057 mm. in length, and are soHd structures, 

 but easily broken up into a mass of minute fragments. The 

 ectoderm is also furnished with a number of flask-shaped 

 unicellular glands, the ducts of which appear to perforate the 

 homogeneous ectodermic tissue. Beneath the ectoderm are 

 three muscular layers — viz. (2) an outer layer of circularly 

 disposed muscle fibres ; (3) a layer of longitudinal muscle 

 fibres ; (4) an inner layer of diagonal muscle fibres. The 

 muscular layers are more strongly developed in the anterior 

 than in the posterior half of the body, the diagonal muscles 

 being found only in the anterior half, whilst the circular 

 muscles diminish notably in size towards the posterior end. 

 On the other hand, the longitudinal muscles are best de- 

 veloped in the hinder end of the body. The oral and ventral 

 suckers are provided with special muscular coats and also with 

 special protractor and retractor muscle bands. The individual 

 muscle fibres in Distomum hepaticum are from '06 to "09 mm., 

 long and about '003 mm. in diameter. A fully formed muscle 

 fibre consists wholly of refractive contractile substance without 

 a trace of nucleus or protoplasm, but in developing fibres there 

 is a mass of granular protoplasm containing a nucleus lying on 

 one side of the fibre at about the middle of its length, recalling 

 the condition found in the muscular fibres of Obelia. 



The alimentary tract of Distomum is lined by a single 

 layer of more or less columnar epithelial cells, which, taken 

 together, constitute the endoderm. The muscular layers, the 



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