ON WANDERING CELLS IN ECHINODERMS. 97 
dermis are seen scattered small rounded and irregularly 
shaped cells containing pigment-granules; these pigment- 
granules are mostly of small size, and have the same appear- 
ance as those seen within the epidermal cells. Occasionally 
one can trace fine processes from these cells up into the epi- 
dermal layer, and here and there one seees a rounded cell 
apparently identical with these pigment-cells between the 
bases of the epidermal cells (fig. 7). The pigment-granules 
either give a dark brownish or a brighter orange-brown colour 
when observed en masse under the microscope; these two 
forms are seen both in wander-cells about the tissues in the 
mantle, &c., and in the epithelial cells themselves. Whence 
these pigmented wander-cells come from I have not yet deter- 
mined, either from the literature or actual investigation of the 
subject, but the description Grobben (83) gives of certain pig- 
ment-cells occurring in the pericardial gland of Arca Noe is 
suggestive. Kowalevsky (40) has shown that this organ has 
some value as an excretory organ. 
In some cases these pigment-granules are thrown off from 
the epithelial-cells by traversing their outer hyaline portion 
(vide fig: 6),in other cases by actual migration of the wander- 
cells ; the former seems to be the general rule, that is the epi- 
thelium-cells act as middle men, a part which they also play to 
some extent in Vertebrates, and is represented in fig. 6, where 
also the second process is illustrated by the two pigment 
agglomerations marked Z, and Z,-and-Z. 
In some of my sections some granules are seen in the thin 
attached edge of the shell, as well as in the epidermal cells of 
that region, and in wander-cells in the subjacent tissue; it 
seems likely that pigmentation of the shell takes place in this 
manner. F. Miller (No. 50), however, makes no reference to 
pigment, nor does Leydig (46) describe such a process in his 
‘ Histology,’ but the above quotation (vide, p. 23, ante) from 
his more recent paper (45) shows that he has recognised it. In 
an intermediate paper on Gastropods (No. 46) he does not 
actually state that the pigment-cells travel outwards through 
the epidermis, but indicates that they may find their way to 
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