32 Dr Gaskell, The Origin of Vertebrates. [Nov. 25, 



travelled up into the thoracic or prosomatic fused mass of ganglia. 

 In the Scorpions Lankester has shown a further amalgama- 

 tion in the Scorpionini where the 1st branchial appendage as well 

 as the genital operculum is innervated from the medulla oblongata; 

 and yet again a still further amalgamation in the Androctonini 

 where the two foremost branchial appendages are innervated from 

 the medulla oblongata. 



In fact, we see in the passage of the Crustacean into Limulus, 

 and Limulus into Scorpio and Androctonus, the same law of gradual 

 fusion of more and more of the ganglia of the ventral chain to 

 form the medulla oblongata, until at last all the nerves of the 

 branchial appendages arise from these fused ganglia, and the 

 vagus nerves of Ammoccetes result. The same process has con- 

 tinued onwards into the Vertebrates higher than Ammoccetes with 

 the result of the inclusion of vertebrae and spinal nerves into the 

 occipital region of the skull as seen in various Elasmobranchs and 

 Ganoids. 



Further, the solitary position of the glossopharyngeal, separated 

 as it always is from the vagus, is suggestive when we find that in 

 all the scorpions the nerve to the 2nd gill appendage is taken up 

 and no more, and that in Thelyphonus and the Pedipalpi generally 

 there exists only one gill-bearing appendage in addition to the gill- 

 bearing part of the operculum. In other words, the nerves which 

 correspond to the glossopharyngeal and facial nerves are in- 

 variably found in connection with the operculum and a gill- 

 bearing appendage even when the appendages homologous to 

 those supplied by the vagus nerve did not carry gills. 



The history of the heart and ventral aortce. 



Without tracing the modifications of the vascular system 

 throughout the vertebrate kingdom, it will be sufficient to say 

 that the simplification which takes place as we pass down to the 

 Elasmobranchs shows that the heart and conus arteriosus arose 

 from the subintestinal vein which by dividing into two formed the 

 two ventral aortse, the main function of which was to send the 

 blood into the gills for purposes of aeration. 



Further, Paul Mayer in his paper on the origin of the heart 

 and blood vessels in Elasmobranchs, has shown that this appa- 

 rently single subintestinal vein is in its origin double, and that 

 therefore in all Vertebrates the heart and ventral aorta arises from 

 two long veins which are originally situated on each side of the 

 middle line; by the formation of the head-fold these come together 

 ventrally, coalesce into a single tube which forms the subintestinal 

 vein and heart, still remaining double as the two ventral aortae 

 with their branchial branches into each gill, as is well shown in 



