Jellyfish
Most jellyfish do not have specialized systems for osmoregulation, respiration and circulation, and do not have a central nervous system. Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around the mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift.
The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation. A loose network of nerves called a "nerve net" is located in the epidermis. Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species. A jellyfish detects stimuli, and transmits impulses both throughout the nerve net and around a circular nerve ring, to other nerve cells. The rhopalial ganglia contain pacemaker neurones which control swimming rate and direction.
Theodore Holmes Bullock
Bullock published a vast array of papers. Other than the species previously mentioned, he also studied the nervous systems of corals, sea urchins, spirunculids, "Limulus", "Aplysia", starfish, rattlesnakes, rays, sharks, porpoises, sea lions, cuttlefish, catfish, sloths, manatees, salamanders, frogs, turtles, hagfish, crayfish, tuna, ratfish, bats, crabs, octopodes, snakes, rats and humans. In 1965 together with Adrian Horridge, Bullock published the seminal two-volume “bible of invertebrate neurobiology”: "Structure and Function in the Nervous System of Invertebrates".