Telencephalon
Telencephalon in all vertebrates possesses two divisions: pallium (dorsal portion) and subpallium (ventral portion) (Nomura, Kawaguchi et al. 2013)and these divisions may be subdivided into different parts depending on type of radiation or group.
However, the presence of telencephalon homologs in invertebrates has been not reported yet. Nonetheless, in the vertical lobes of some invertebrates brains there are neurons with physiological properties that are dedicated to memory and learning. (Hochner 2010) for example in octopus there are some areas in their brain involved in learning, memory, analysis and etc. which are analogs of the cerebral cortex. (Elwood 2011)
In higher animals, it is indicated that fish are able to learn and perform motor behaviors by using different regions of their forebrain (Demski 2013)that are weakly homologous to the mammalian telencephalon. Lateral pallium and medial pallium of fishes probably have similar function as well as their homologous structures (hippocampus and the amygdala respectively) in spatial (LP) and emotional learning (MP). Although the medial pallium is weakly homologous to the amygdala, these structures basically possess feedforward circuits that perform nociceptive defensive behaviors. (Braford Jr 1995, Northcutt 2006, Northcutt 2008, Braford Jr 2009, Mueller and Wullimann 2009, Northcutt 2011, Maximino, Lima et al. 2013, Janak and Tye 2015). Also, although the function of their dorsal pallium which suggested as homologous to mammalian isocortex and transition cortex is mostly unknown but it might have a role in avoidance response in trace avoidance tasks. (Portavella, Torres et al. 2004, Vargas, López et al. 2009)
In agnathans radiations such as lampreys and hagfishes telencephalic structure consists of olfactory bulbs, cerebral hemispheres which are paired evaginated, and a telencephalon medium. According to Heier (Heier 1948) ascending dorsal thalamic and hypothalamic connections with the caudal pallium of lampreys exist in these fishes . (Northcutt 1981)Although there is a debate that either cerebral hemisphere arose first or olfactory bulb (Northcutt 2002) but most of these characters were probably present in their common ancestor.
Cartilaginous Fishes, which are believed to have given rise to amphibians, the dorsal pallium is subdivided into inner and outer laminae and stretch through the midline to form an interhemispheric bridge(Northcutt 1981). Also in these fishes the ascending pathways have inputs from the thalamus and descending pathways which have protrusion to the thalamus, optic tectum, medulla, and even cervical spinal cord have been reported.(Ebbesson 1980)
The variation in telencephalon of ray finned fishes is as much as land vertebrates but unlike other vertebrates, their hemispheres are not formed by inversion and evagination (Northcutt 1981). These different processes make prominent changes. For instance, in actinopterygian fishes, unlike other groups, the telencephalic hemispheres are separated by a unique ventricle instead of internal ventricles and it is because of their telencephalon undergoes eversion process(Salas, Broglio et al. 2003) .
The cortex area in fishes is initial so that this area in elasmobranchs is formed by the large groups of the cells with thalamic inputs and also in teleost it develop better and hemispheres possessing complex projections to the diencephalon and midbrain(Northcutt 1981).
Probably, the most notable variation in telencephalon of land vertebrates happened in amphibian reptile cline rather than crossopterygian-amphibian because the organization and development of telencephalon in amphibians is almost similar with primitive sharks and polypteriform fishes. (Laberge, Mühlenbrock-Lenter et al. 2006)
The small telencephalon of amphibians possesses few but large neurons which have dense periventricular cellular layer in most parts. (Laberge, Mühlenbrock-Lenter et al. 2006) The pallium in amphibians is subdivided into four parts including dorsal, lateral, and medial pallial sheets, and a pars lateralis of the amygdala(Northcutt 1981). Amygdala is diagnosed as a neural center for emotional learning(Laberge, Mühlenbrock-Lenter et al. 2006).This structure in amphibians include a pallial pars lateralis and a subpallial pars medialis that imply the development of emotional capacity in amphibians(Northcutt and Kicliter 1980, Moreno and González 2003) . The ventral part of the ventral pallium in amphibians might be the homologous of amygdalar (Northcutt and Kicliter 1980, Marı́n, Smeets et al. 1998)
The pallium in reptiles is divided into cortex and DVR (dorsal ventricular ridge)(Nomura, Kawaguchi et al. 2013). In living reptiles and birds, telencephalon has a heart shape formation in dorsal view due to the expansion of lateral wall (DVR). However, in cynodonts (mammallike reptiles) cerebral hemispheres are long and narrow and do not similar to living reptiles or mammals hemispheres (Northcutt 2002).
One of the considerable differences between reptiles and birds with mammals’ telencephalon is that in birds and reptiles the thalamic projections extend outside the telencephalon but in mammals they extend inside telencephalon with internal capsules. These projections are recognized as the routes of sensory information such as auditory information (Vanderhaeghen and Polleux 2004, Nomura, Kawaguchi et al. 2013).
Birds possess a large pallium that includes most of their forebrain. the mammalian neocortex is homologous to these parts in terms of its shared pallial identity deriving from common ancestry (Jarvis, Güntürkün et al. 2005). The absence of laminated forebrain in birds is the main difference between mammalian and avian forebrain. (Güntürkün 2005) Many studies suggest that a region in the dorsolateral forebrain of pigeons (nidopallium caudolateral: NCL) is comparable with the mammalian PFC (Mogensen and Divac 1982) but, they do not seem to be homologous of each other and probably represent a case of evolutionary convergence (homoplasy).
Most of the cognitive abilities of mammals and birds depend on the prefrontal cortex and the nidopallium caudolateral. Since the nonlaminated NCL is able to generate the same level of functions and complexity as the PFC, lamination cannot be a structural requirement for higher cognitive functions. (Güntürkün 2005)
Additionally, telencephalon in mammals is the most complex region of their brain and shows great diversity of neuron types. Their telencephalon is divided into different sections such as cerebral cortex and basal ganglia which play roles in consciousness emotion, complex behaviors and cognition. (Guillemot 2005)