DISTOMUM HEPATICUM 7 



epithelial and muscular tissue in which the following layers 

 may be distinguished: (i) Externally a thick cuticle, 

 homogeneous in its outer and superficial but vertically striated 

 in its deeper portion. This cuticle swells up in water, and 

 in it are imbedded the chitinoid scales already mentioned. 

 The scales, though they appear to project from the surface of 

 the animal, are really contained in as many pocket-like cavities 

 of the cuticle, and their projecting ends are covered over by 

 a thin layer of the homogeneous cuticular tissue. The scales 

 vary in length from '036 to "057 mm. and are solid structures, 

 but easily broken up into a mass of minute fragments ; (2) the 

 ectoderm proper, consisting of a thin sheet of protoplasm 

 below the cuticle continuous with the attenuated outer ends 

 of a number of flask-shaped nucleated cells, the irregularly 

 shaped swollen inner ends of which lie below the muscular 

 layers immediately to be described. The protoplasmic layer 

 and the flask-shaped cells taken together represent an 

 ectoderm, of which the component cells have sunk down into 

 and are imbedded in the underlying muscular layers j (3) an 

 outer layer of circular muscle fibres; (4) an inner layer of 

 longitudinal muscle fibres ; (5) an inner layer of diagonal 

 muscle fibres traversing a parenchyma composed of branching 

 cells, among which are numerous scattered bladder-shaped 

 cells. 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 



