It should be added that Professor Elliot Smith has pointed out a certain peculiarity in its commissures whereby the brain of the diprotodonts differs markedly from that of the polyprotodonts From Flower, Quart.
Osborn, " The Origin of the Corpus Callosum, a contribution upon the Cerebral Commissures of the Vertebrata," Morphol.
The brain-lobes are generally four in number, a ventral and a dorsal pair, respectively united together by the above-mentioned commissures, and moreover anteriorly interfusing with each other, right and left.
Places it has sunk into the con Passing from the middle line outwards they are - (i.) the median pallial nerve to the middle of the dorsal mantle; (ii.) numerous small nerves - the circum-oesophageal commissures - which pass round the oesophagus to the chief arm-nerve or supra-oesophageal ganglion; (iii.) the under arm-nerve to the lophophore and its muscles; (iv.) the lateral pallial nerve to the sides of the dorsal mantle.
The wide divarication of the lateral cords in the prosoma and their connexion by transverse commissures, together with the " attraction " of ganglia to the prosomatic ganglion group which properly belong to hinder segments, are very nearly identical in the two animals.
The pedal cords are connected by commissures, and the pedal and pleural of each side are similarly connected.
The perioesophageal nerve-ring of Chaetopoda and Arthropoda is represented, not by the collar first mentioned in the above description, but by the commissures connecting the cerebral and pedal ganglia.
There is a welldeveloped brain dorsal, to the mouth; this gives off a pair of oesophageal commissures, which surround the oesophagus and unite in a median ventral nerve-cord which runs between the longitudinal muscles to the posterior end of the body.
As regards their innervation an apparent exception is found in the case of A pus, where the nerves to the antennules arise, behind the brain, from the oesophageal commissures, but this is, no doubt, a secondary condition, and the nerve-fibres have been traced forwards to centres within the brain.
In the primitive Phyllopoda, and less distinctly in some other orders, the nerves supplying the antennae arise, not from the brain, but from the circum-oesophageal commissures, and even in those cases where the nerves and the ganglia in which they are rooted have been moved forwards to the brain, the transverse commissure of the ganglia can still be traced, running behind the oesophagus.