STUDIES ON THE TURBELLARIA. 429 
an independent layer, but that it is to be looked upon as a 
somewhat modified portion of the epidermis. 
The cilia, which cover both surfaces, have an average 
length of about ‘008 mm. Here and there are longer flagella, 
which appear to be non-vibratile, but are often passively 
moved by the cilia at their bases. Hach cilium consists of 
two segments, a proximal, straighter and stiffer, and a distal, 
more flexible and whip-like. Vertical lines in the protoplasm 
of the epidermis have the appearance of continuations of the 
cilia inwards. 
In A. cinereus von Graff (15, p. 4) describes the epi- 
dermal cells as resting on the underlying muscular layer by 
a base which is often branched, the presence of the branches 
with the spaces between them giving an alveolar appearance 
to the inner zone. This appearance was not observed in 
Heterocherus. 
In the case of A. cinereus von Gratf states that, in 
addition to the true epidermal cells, and the gland cells with 
their ducts, the epidermal layer also comprises a number of 
cells which he terms interstitial cells. In Heterocherus 
these do not occur, at least in the same form; but stellate 
pigment cells (fig. 4) extending over the dorsal surface in 
mature specimens, and often spreading to some extent over 
the ventral, may represent them. ‘The processes of these 
stellate cells are usually completely united into a network, 
and in most parts, at least in mature specimens, their cellular 
nature becomes obscured, especially since nuclei are very 
difficult of detection among the dense pigment, and it is, 
besides, very hard to distinguish them with certainty from 
the ordinary nuclei of the epidermis. In some cases, however, 
more particularly in immature specimens, it becomes manifest 
that the pigment is contained in stellate cells, the processes 
of which anastomose to form a network, extending through 
the epidermal layer towards its deeper surface. 
‘Though the occurrence of epidermal pigment appears to be 
somewhat exceptional in the Turbellaria, yet bodies of the 
same nature as those here regarded by me as stellate pigment 
