172 THE ORIGIN OF VERTEBRATES 
of the ventral aorta (V.A.), and dorsally to the mid-line between the 
dorsal aorta (D.A.) and the notochord. 
The close relationship of the constrictor muscle to the carti- 
laginous 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 (mu, Fig. 66) of the 
Limulus appendage, especially when the homology of the Ammoccetes 
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 Ammoccetes branchial segment as more likely 
to have been derived from dorso-ventral muscles which belonged 
originally tu 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 Ammoccetes, while dorso- 
ventral appendage-muscles have been retained, 7.c. the exact reverse 
to what has taken place in the air-breathing scorpion. 
Iam 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, 
ie. 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 muscu- 
lature of the corresponding appendages, is, speaking generally, dorso- 
ventral in direction in Ammoccetes and of the same character through- 
out; 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 
Ammoccetes most sharply defined from all the other iuscles of 
the body. They form the great dorsal and ventral longitudinal 
