It has, however, been alleged that this cavity is formed by a pair of mesoblastic somites (N.
In the middle of the body, where the limits of the somites can be checked by a comparison with the arrangement of the nephridia and the gonads, and where the ganglia are quite distinct and separated by long connectives, each ganglion is seen to consist of six masses of cells enclosed by capsules and to give off three nerves on each side.
The head of an insect carries usually four pairs of conspicuous appendages - feelers, mandibles and two pairs of maxillae, so that the presence of four primitive somites is immediately evident.
The number of limb-bearing somites in the insectan head is thus seen to be seven.
The brain innervates the eyes and feelers, and must be regarded as a " syncerebrum " representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes.