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.
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.
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.
The circulatory system of Carinella is considerably different, being more lacunar and less restricted to definite vascular channels.
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.