342 T-isWis iii';xi;v (;(»Li(iii. 



liairs or other such structures. A fine radial striation is, 

 however, readil}^ observable in favourable sections (fig. 17). 

 The thickness of tliis layer is from 1 |u to 1*5 fi. The comi- 

 dial layer easily becomes detached, and is then sloughed. 



The homogeneous layer is -3 jn thick on the average. Unlike 

 the outer layer, it hardly stains with haematoxylin, but takes 

 orange G or eosin readily. It appears (when fixed by 

 " Zenker ") to be quite structureless, except on the suckers, 

 but impregnation with silver shows a definite structure also 

 elsewhere. I have not been able to find any structures which 

 can be compared to Minckert's (1906) trophopores, tropho- 

 porelles, neurophyses, or neuropores in the homogeneous layer 

 of Avitellina centripunctata (Rivolta), nor any signs of 

 pores running from surface to surface through this layer. 

 However, in specimens impregnated with silver there are 

 minute black granules scattered through the homogeneous 

 layer, not quite evenly distributed, but more crowded towards 

 the basal membrane, more scattered towards the comidial 

 layer (fig. 18). I have observed a similar structure of the 

 cuticula in Dipj^lidium caninum, L., fixed with Zenker, 

 stained with iron-hseraatoxylin, and counter-stained with 

 eosin ; here the homogeneous layer presents exactly the same 

 appearance as that of Avitellina when fixed with silver 

 nitrate. The granules must therefore represent a finest 

 structure of the cuticula, and are pi-obably not merely 

 artefacts (silver precipitates), as 1 was at first inclined to 

 believe. 



Certain modifications of the homogeneous layer are, how- 

 ever, invariably present in the cuticula of the suckers. These 

 remain constant whether the specimen was fixed with Zenker, 

 silver nitrate, or formalin. Tiie comidial layer presents no 

 contrasts to elsewhere, but the inner half of the homogeneous 

 layer appears spongy or reticulated (fig. 17). The reticula- 

 tions are formed by fibres running mostly at right angles to 

 the homogeneous stratum, and forming numerous anastomoses 

 amongst themselves. These fibres may be continuous with 

 the parenchyma fibres of the suckers, which they much 



