Anteriorly it finally communicates with the lacunae just mentioned, which surround the oesophagus, bathe the posterior lobes of the brain, pass through the nerve ring together with the proboscidian sheath, and are generally continued in front of the brain as a lacunar space in the muscular tissue, one on each side.
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.
Frequently, also, a considerable differentiation of vegetative tissue occurs in the wall of the spore-capsule itself, and in some of the higher forms a special assimilating and transpiring organ situated just below the capsule at the top of the seta, with a richly lacunar chlorophyllous parenchyma and stomata like those of the wall of the capsule in the Anthocerotean liverworts.
The circulatory system of Carinella is considerably different, being more lacunar and less restricted to definite vascular channels.
There are no lacunar blood spaces with ill-defined or absent walls except for a sinus surrounding the intestine, which is at least frequently present.
This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through whicF air can pass to the deeper lying tissues.