50 POST-LARVAL STAGE OF ARENICOLA MARINA. 
In the first few bundles there are only three cheetze ; the number in- 
creases as we pass backwards, till in the hinder somites eight or nine 
cheetze constitute a bundle. Both the dorsal and ventral cheete 
differ from those occurring in the adult, in which the dorsal cheetz 
present a series of small processes on each side (see Cunningham, 
Trans. Ed. Roy. Soc., xxxiii, 1888) ; whilst the ventral ones, as my 
fig. 3 shows, are without the smaller prong. Naturally the size of the 
cheete differs, some idea of which difference will be conveyed by a 
comparison of figs. 2 and 3. 
Of the thirteen pairs of gills in the adult, the present post-larval 
stage shows in profile only six pairs, situated on Somites 14—18. 
But I believe I can detect other gills on succeeding somites in the 
mounted specimen; however, it is difficult to be certain of the 
number. In the adult the first gill, which is quite small, occurs on 
the ninth somite (seventh cheetigerous), and the last in the nineteenth 
cheetigerous (7. e. the twenty-first somite). Thus the gills make their 
appearance from behind forwards. Hach gill is at present merely 
a small somewhat conical papilla or eversion of the epidermis, con- 
taining a potential cavity entirely occupied by a looped blood-vessel 
(see fig. 5). The appearance suggests that these gills of Arenicola 
are special structures, and not, as in Hunice and other free-living 
Polycheetes, modifications of the dorsal cirrus, as there is no trace of 
sensory hairs, which are present even in quite early stages in the 
development of cirri—for instance, in Polydora. 
The epidermis is at this stage a single layer of cells, and varies, as it 
does in the adult, in different parts of the somite. In the non-glandular 
band (figs. 4 and 7) the cells whose outlines are not recognisable are 
flattened, and the nuclei are oval with their long axes parallel with 
the surface of the body. In the glandular band, however, the 
epidermis is thicker (fig. 5), and consists of narrow, deeply staiming 
cells, compressed between large gland-cells. The nuclei of the 
former are small and circular, and of the latter compressed against 
the side of the cell. Viewed from above, the epidermis presents the 
appearance of fig. 7. 
Wirén (Kongl. Svensk. Vet. Akad. Handlingar, 22, pl. i, figs. 
1—23) has described a similar difference in the epidermis cells 
between the ridges and those constituting the ridges in the adult. 
Probably these glandular bands of. the post-larval stage become 
broken up into the polygonal areas or ridges of the adult. 
I would here call attention to the strongly marked grooves, each 
followed by a distinct ridge, in the anterior somites ; in this ridge the 
cheetee are inserted. ‘This is true of both the post-larval stage and of 
the adult, and one would at first sight regard these grooves as 
intersegmental grooves, so that the cheetee appear to have the rather 
