Thalia democratica have a worldwide distribution, they inhabit all ocean basins as successful gelatinous mesoplankton (Madin, 1995). Behaviours which have led to such successes are dependent on sensory cues from the external environment. The reception of both physical and biological parameters are essential for success within the pelagic ocean habitat T. democratica inhabit. 

T. democratica has a complex central nervous system underpinning all of the basic functions of the animal. The two major sensory organs connecting with this system are situated mid anteriorly along the dorsal surface. These arethe dorsal tubercle and the dorsal ganglion (fig 1) (Madin, 1995). The dorsal ganglion is a brain-like structure which has been used asan evolutionary model to represent the pre-chordate brain. Situated dorsally atop of the ganglion is a horse shoe shaped eye rudiment.


 T. democratica has a complex central nervous system. The Ganglion has approximately seven pairs of rostral and lateral nerves, one anterior medial nerve, and three pairs of caudal nerves (Lacalli andHolland, 1998), which can be seen extending out of the central ganglionic region in figures 1 and 2. Within the ganglion lie 6 neuronal clusters, 3 on either side of the ganglion along the anterior posterior axis (fig 3)(Lacalli andHolland,1998). These clusters of neurones giving rise to the peripheral nerves are regionally directed such that, nerves from C1 extend forward to oral muscles, nerves from C2 innervate the first muscle band, and nerves from C3 extendposteriorly into the caudal regions.The caudal nervesform attachments with muscles 4 and 5 and other posterior visceral regions (Lacalli and Holland, 1998). Attachment of the peripheral nerves to the muscles are what allow coordinated movement and propulsion of the organism through the water. The complex central nervous system allows T. democratica to exhibit more complex behaviours such as phototaxis, geotaxis, and flee responses to tough stimuli (Madin, 1995).