102 Macieay Memorrau Vouvme. 
the different species: in 7. mznor they are ‘004mm. in diameter, in 7. /heringit 
005 mm., in 7. fasctata 0035 to ‘0075 mm. ; while in 7. guadricornis they reach the 
relatively large size of as much as ‘02mm. in some cases. They are scattered at 
fairly regular intervals over the surface—about ‘02 to ‘04mm. apart in the Australian 
species, more widely separated in 7. /heringzt. 
These two layers—the cuticle and the epidermis—are, as already mentioned, 
distinguishable in the living condition, the cuticle appearing as a well-defined narrow 
clear band, superficial to the more granular-looking epidermis ; they are separable, 
though not always readily, by maceration. In sections stained in various ways they 
are clearly differentiated, especially if picrocarmine be the staining agent employed. 
In specimens that have been fixed by means of osmic acid solutions the differentiation 
is usually not so clear—the acid darkening both layers and preventing staining agents 
from producing a differentiating effect. 
Underlying the epidermal layer is a very definite, homogeneous, non-nucleated 
basement membrane (PI. x. figs. 1, 2, 7 and 8, 6. m.; Pl. xiv. fig. 4, 6. #.), usually of 
nearly the same thickness as the epidermis, but varying a good deal in this respect, in 
accordance apparently with the condition, as regards contraction or relaxation, of the 
specimen or the part. Under ordinary conditions it is thicker on the dorsal surface 
than on the ventral; excessively thin on the tentacles and ventral surface of the 
sucker. This layer is present in all the species 1 have examined ; but is absent 
according to Weber in 7. Semfert, the epidermis in that species resting directly on 
the layer of circularly arranged muscular fibres. I have called it homogeneous, as in 
the vast majority of sections, and in macerated and teased preparations, it has the 
appearance of being so; but in one series of sections of 7. Dendy prepared by 
Flemming’s method it presents (Pl. x. fig. 8, 4. 7.) a very minutely reticulate struc- 
ture only distinguishable under a high power. This is not in any way comparable to 
the complex arrangement of cells and processes described by Lang as characterising 
the basement membrane in the Polycladide.* By many staining processes the 
basement membrane is not affected, and thus stands out in marked contrast, as I have 
endeavoured to show in fig. 2, to the epidermis on the one side and to the layer of 
circular muscular fibres on the other ; but some carmine solutions stain it intensely— 
much more so than the epidermis. When a specimen which has been stained with 
picrocarmine is teased, the basement membrane is usually readily discernible, owing 
to its being brightly stained and exhibiting a firmer consistency than the other 
layers. Adhering to its inner surface in such preparations are found scattered fibres 
partly belonging, apparently, to the circular layer, partly to the muscular system of 
the parenchyma. It is perforated by the pore-canals, but these are difficult to see, 
* “Die Polycladen,” p. 64. 
