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 (mi, 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 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 Ammoc<etes, 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, Vllth, 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 

 Ammoca'tes most sharply defined from all the other muscles of 

 the body. They form the great dorsal and ventral longitudinal 



