Anatomy of finfish and shellfish

Unit 7 - Nervous system in fishes

7.1.1. The Brain
Generally speaking fish have small brains in relationship to their overall body weight. Elasmobranchs (Sharks and Rays) in general have a slightly larger brain for the same body mass as Teleosts (Bony Fish), however there is great variety within the teleosts scientists have learned something quite surprising about the Elephantnose Fish (Gnathonemus petersii).
The brains of cyclostomes (Hagfish and Lampreys) are simple but specifically evolved to suit their lifestyles. For instance the optic lobe is well developed in the visually oriented Lampreys but indiscernible in the blind Hagfish. In both however the medulla is large and the cerebellum small. Together the cerebellum and the medulla make up the hind brain.
The medulla controls the operations of the inner organs such as heart rate, blood pressure, digestion and waste disposal. It is also a relay centre for many nerves sending messages to and from the mid and or forebrain.
The cerebellum controls motor co-ordination (but it does not initiate motor activities). This means it controls the timing and interaction of muscles once a muscular action has been initiated. The cerebellum is also important in maintaining equilibrium.
The mid-brain of a fish consists mostly of the optic lobes, which vary greatly in size between species in accordance with their dependance on sight, and in some species the optic lobes may be so large they completely cover the forebrain. In fish the mid-brain is important in sorting out incoming information and it is also the main centre of learning (whereas in mammals it is the forebrain that is the main centre of learning).
The forebrain of fish is dominated by the olfactory lobes which extend forwards and may be placed at the end of stalks. These olfactory lobes are large in the cyclostomes and very large in the elasmobranchs reflecting the importance of smell to these to groups of fish. The teleosts, for whom sight is often the most important sense have smaller olfactory lobes.
In many elasmobranchs and some teleosts there exists a cerebrum or pair of cerebral hemispheres These also seem to be predominantly involved with the sense of smell (in mammals the cerebrum is much larger and involved in planning and learning). The pituitary also arises out of the forebrain, it plays and important role in the regulation of metabolism.
A fish's brain never completely fills the cranium, the cavity in the skull where it lies protected. The remaining space is filled up with a gelatinous material. Finally as in all vertebrates the brain, plus the gel, are surrounded by a membrane that helps keep foreign matter and micro-organisms from contacting this most important organ.
The forebrain in fishes denoted to the reception elaboration and conduction of smell impulses. The mid brain consists of the optictectum and all vision senses are received here. This optic tedium is great functional importance in the central nervous system (CNS) and has several layers of nerve cells.
Hind brain controls the swimming equilibrium, maintenance of muscular tomes and orientation in space in the anterior most regions. The brain divisions have developed as the sensory integrations. It is the centre to which lead the sensory nerves except those of smell (I) and sight (II)