The close relationship of the constrictor muscle to the cartilaginous branchial bar does not favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the scorpion. It is, however, directly in accordance with the view that this muscle is homologous with one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle (_m_1_, Fig. 66) of the Limulus appendage, especially when the homology of the Ammocoetes branchial cartilage with the Limulus branchial cartilage is borne in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the Ammocoetes branchial segment as more likely to have been derived from dorso-ventral muscles which belonged originally to a branchial appendage, such as we see in Limulus, than from dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in Limulus and scorpion. In other words, I am inclined to hold the view that the somatic dorso-ventral muscles have disappeared in this region in Ammocoetes, while dorso-ventral appendage-muscles have been retained, _i.e._ the exact reverse to what has taken place in the air-breathing scorpion. I am especially inclined to this view because of the manner in which it fits in with and explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates into parietal and visceral, such a division has received general acceptance and, as far as the head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van Wijhe, _i.e._ to the somatic and splanchnic striated muscles according to my nomenclature. Of these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammocoetes and of the same character throughout; the somatic musculature, on the other hand, is clearly divisible, in the head region, into two sets--a spinal and a cranial set. The somatic muscles innervated by the spinal set of nerves, including in this term the spino-occipital or so-called hypoglossal nerves, are in Ammocoetes most sharply defined from all the other muscles of the body. They form the great dorsal and ventral longitudinal {173}body-muscles, which extend dorsally as far forward as the nose and are developed embryologically quite distinctly from the others, being formed as muscle-plates (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic trunk-muscles, and must, therefore, form a separate group to themselves.