The vascular system itself is quite peculiar, consisting of lacunae and channels destitute of endothelium, situated within the thickness of the basementmembrane of the body-wall, of the gut-wall and of the mesenteries.
Certain free mobile cells within the body, such as blood-leucocytes, as well as others which are fixed, as for instance the endothelium of the hepatic capillaries, have the property of seizing upon some kinds of particulate matter brought within their reach.
Thus, while increased pressure in the blood or lymph vessels may be one factor, and increased permeability of the capillary endothelium another, increased osmotic pressure in the tissues and lymph is probably the most important in the production of dropsy.
In the latter case they invade cells of a leucocytic or phagocytic character as a rule; Leishman's form is particularly abundant in large macrophageal cells originating from the vascular endothelium of the spleen (fig.
Nevertheless the filament is a complete tube formed of chitinous substance and clothed externally by ciliated epithelium, internally by endothelium and lacunar tissue - a form of connective tissue - as shown in fig.
The ventricle and auricles of Anodonta lie in a pericardium which is clothed with a pavement endothelium (d, fig.
The structure of the corpora cavernosa consists of a strong fibrous coat, the tunica albuginea, from the deep surface of which numerous fibrous trabeculae penetrate the interior and divide it into a number of spaces which are lined with endothelium and communicate with the veins.
Probably the lining endothelium of the blood-vessels as well as other tissues of the body participate in the production of anti-substances.