DEVELOPMENT OF PEEIPATUS CAPENSIS. 
763 
The interior of the sac is lined with large flat endothelial cells, with a polygonal out- 
line, and resembling the well-known endothelial cells of the frog (Plate LXXIII. 
fig. 6). The secreting tubes of the gland itself (Plate LXXlll. fig. 7) are covered 
externally with a layer of hexagonal plaster-cells (not shown in the figure), and in- 
ternally are lined with a thick layer of slime-cells, which are elongate, often pear- 
shaped, disposed transversely in the gland-tubes, with their narrower bases resting on 
its wall and their pear-shaped ends projecting into its cavity. The cells have the usual 
granular structure and a very distinct nucleus and nucleolus. The riper cells become 
transparent and almost free from granules. An irregular space leading down the 
axes of the tubes is filled with fluid containing fine granules. The slime-cells probably 
swell gradually to the transparent condition, and then bursting, discharge their contents 
into the cavity of the gland. The glands exhibit all the viscous properties of the fluid 
ejected by the animal, and are so sticky as to be of considerable hindrance in the 
dissection of the animal in the fresh condition or under water. The oral papillae in 
which the glands open, as will be seen in the sequel, are modifications of the second 
pair of body-members of the embryo. The glands are probably homologous with the 
silk-glands of caterpillars and the poison-glands of Scolopendra. There are no 
excretory glands representing the Malpighian tubes of insects and Myriopods in 
Peripatus. 
Fat-bodies. — The lateral bodies (x, Plate LXXII. fig. 1) were described as two canals 
imbedded in the muscular wall of the body, which might perhaps belong to the 
vascular system (Claus, Zoologie, p. 387). These bodies vary very much in different 
specimens. In some they are narrow and terminate about the middle of the body ; 
in others they are much longer and broader, and stretch to the very end of the body. 
They are composed of small round oily particles, and apparently are homologous with 
the fat-bodies of other Tracheata. 
Circulatory System. — The dorsal vessel, when examined in the fresh state or in glyce- 
rine, appears to be of very simple structure. As in insects, it is imbedded in fatty tissue. 
I have been unable as yet to see any valvular arrangements in it or system of trans- 
verse fibres. I intend to examine its structure more carefully by means of sections See. 
When the vessel is dissected away from the body-wall, a series of openings are seen in 
the middle line of the body-wall, directly above it, exactly like those which exist on 
either side of the ventral wall of the body along the line of origin of the feet. The 
ventral openings were described by Grube. They are figured in Plate LXXII. fig. 2, 
where they are seen situate on either side, immediately externally to the ventral 
nerve-cords ( v.c , v.c). 
Respiratory System. — The tracheal system in Peripatus renders itself conspicuous at 
once when the animal is opened in the fresh condition. The tracheae have the charac- 
teristic pearly lustre given by all fine transparent tubes when filled with air and seen 
by reflected light. In specimens which have become saturated with spirit, the air 
becomes entirely removed from the vessels, and they are consequently hardly to be seen 
