Hemolymph:
Haemolymph nodes are enigmatic structures that in both histological and functional terms appear to be intermediate between ordinary lymph nodes and the spleen. Whether they occur in man is debatable, but they certainly occur in the pig, sheep, horse and rodents, including the rat (Sakita et al., 1997; Kazeem et al., 1982; Kazeem and Scothorne, 1982; Hogg et al., 1982). Turner (1969) reported a detailed study of the vascular tree of the haemal nodes of the rat. Haemolymph nodes are characteristically dark red or brown in colour, and have been reported at the edge of the thymus, between the spleen and the pancreas, and bilaterally close to the aorta near the origin of the renal arteries. Haemolymph nodes differ from ordinary lymph nodes in that their lymphatic sinuses contain many red blood cells, which are thought to pass through the walls of rather permeable venous blood vessels. Sakita et al. (1997) suggested an open circulation (see discussion of the spleen) with blood cells entering the lymphatic sinuses by crossing their walls. The function of these unusual nodes is obscure. [Science Direct]
Noted in “Devious Parasite Grants Host the Gift of Eternal Youth, But For a Price,” Spooky, Oddity Central:
Temnothorax-nylanderi is a relatively common species of small ants that live in forests throughout Central Europe. They form small colonies on the forest floor, inside acorns or wooden branches, and most importantly, they serve as an intermediate host for the tapeworm Anomotaenia brevis. Up to 70 parasitic larvae can survive in the hemolymph, the body fluid of insects, but instead of competing for resources with their hosts and slowly killing them, the parasites appear to extend their lives considerably, possibly even indefinitely.
[h/t TW]
