38) from six to sixteen segments are clearly marked by ridges and grooves in the metasomatic tagma, whilst in Illaenus the shield so formed is large but no somites are marked out on its surface.

The Arthropod head is a tagma or group of somites which differ in number and in their relative position in regard to the mouth, in different .....

But in higher Crustacea the cephalic " tagma " is extended, and more somites are added to the fusion, and their appendages adapted as jaws of a kind.

The genital apertures are placed on the first somite of the second tagma or mesosoma.

Eleven somites follow these, forming the abdominal " tagma," giving thus 1 Embryological evidence of this is still wanting.