CeSP
Cerebral Serotonergic Posterior (CeSP) cluster is a group of 5 serotonergic neurons in Tritonia, Melibe, and other nudibranchs.

Serotonin immunohistochemistry shows the locations of the DSIs (circled).
CeSP-A refers to three of the 5 neurons in this cluster that have an axon that projects to the contralateral pedal ganglion. The Dorsal Swim Interneuron (DSI) is an example of a CeSP-A.
See also Melibe CeSP-A