Atolla jellyfish
The body of "Atolla wyvillei" is a bell shape and it has several moderately long tentacles along the bell. The bell is around 20–174 mm in diameter. It also has a single long tentacle called a hypertrophied tentacle. This hypertrophied tentacle has several purposes, including aid in predation and aid in reproduction. These jellyfish do not have a digestive system, a respiratory system, a circulatory system or a central nervous system. Not having a central nervous system means that it lacks a brain.
Aglantha digitale
"Aglantha digitale" is unique among known jellyfish in having giant axons in the subumbrella (the concave inner surface of the bell) which are involved in a rapid escape response. Normally the hydrozoan swims by slowly pulsating its bell, movements produced by muscle contractions which eject water through the velar opening. However, when the animal is stimulated by manipulating the bell margin, or by squeezing or tugging a marginal tentacle, it responds very rapidly, producing up to three vigorous contractions that propel it a distance of about five body lengths. In its pelagic habitat, this hydrozoan finds itself among relatively fast moving crustaceans, fish larvae and arrow worms in a jostling diurnal vertical migration, and the escape response may help it to avoid damage to its fragile tissues. Observations from submersible craft have shown the hydrozoan behaving in this way.