Nervous system of the leech
The central nervous system of Hirudinia is a segmented system which derives from five embryonic lineages. The lineages are M, N, O, P, and Q. The segmentation of the leech's central nervous system is characterized by fissures which form columns of founder cells along the nerve cord. Some of these founder cells are very unique- so unique that they are only found in the leech and aren't even shared by a common ancestor! Most of the nervous system's functions are shared within a relationship between the N lineage and the leech's central nervous system. Since most of the leech's nerve fibers are concentrated around the N lineage, this would make a lot of sense (considering that the nerve fibers are what connect the lineages to the CNS). In fact, the only place in which the process of segmentation takes place is the N lineage. The connectivity of these 5 lineages to the CNS is what allows the leech's nervous system to survive as a whole.
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