Agnatha was an aquatic lifeform native of the planet Jezero. A career diplomat, Agnatha was Jezero's ambassador to the Federation in the 23rd century. As an aquatic lifeform with a long, eel-like, tentacled body, Agnatha required a life support tank at all times while aboard Federation starships and starbases. Agnatha was able to communicate with those outside of his tank by the use of a specialized, built-in, universal translator unit.
In 2269, Agnatha boarded the USS Enterprise on a diplomatic mission to Detente Station One for peace talks with the Romulan Star Empire. Agnatha, in particular, was one of a group of ambassadors that objected to the continued discourse with the Romulans, on the basis of well-known Romulan duplicity and militarism.
Unfortunately, Agnatha was the first of several diplomats murdered en route to the conference by Dr. M'Benga, who was under the control of alien spores. Agnatha's tank was ripped from its bolts on the floor, tipped over, and shattered. Investigation of the scene revealed that Agnatha had been gripped by a telepathically-transmitted fear and thrashed about, doing some of the work of damaging his own tank. (TOS novel: Death's Angel)