1895.] Dr Gaskell, The Origin of Vertebrates. S3 



the case of Ammoccetes. The heart, then, of all Vertebrates, unlike 

 the invertebrate heart, is formed from the coalescence of two main 

 venous trunks or sinuses situated ventrally, its original purpose 

 being to send blood into the gills for the purpose of aeration. One 

 step further leads to the conclusion that these two longitudinal 

 venous sinuses are the two longitudinal venous collecting sinuses 

 of Limulus and Scorpio. In both cases they form two large ven- 

 trally situated blood vessels from which the blood passes to the 

 gills, and, further, in both cases the walls of these two sinuses are 

 connected with the pericardium by a system of transparent muscles 

 described by Milne Edwards and named by Lankester veno-peri- 

 cardiac muscles. 



These muscles are hollow, both near the vein and near the 

 pericardium, so that the blood in each case fills the cavity, and 

 as they contract with the heart, that part of them in connection 

 with the venous collecting sinus already functions, as pointed out 

 by Milne Edwards and Blanchard, as a branchial heart. 



The ventral aorta? of Ammoccetes correspond exactly in position 

 and in function with these two venous collecting sinuses in Scor- 

 pion, and therefore in Limulus; and we see in Ammoccetes the 

 position of the original dorsal heart represented by that extra- 

 ordinary column of peculiar tissue containing fat which is situated 

 in the middle line between the spinal canal and the surface. 



The further relation between the vascular and lymphatic sys- 

 tems of Ammocoetes and the vascular system of Scorpio to- 

 gether with the consideration of the pronephros, will be considered 

 later. 



The history of the Alimentary Canal. 



The history of the vertebrate alimentary canal is the history of 

 nature's great mistake and how she rectified it ; for at the time 

 when Vertebrates were formed, the lines upon which evolution was 

 working led straight to an increasing antagonism between the 

 increase of brain power and the sufficient feeding of the organism ; 

 for the increase of brain power meant the increase of connections 

 between the supra-cesophageal ganglia and the lower nerve 

 centres, meant therefore an increase of nervous matter around 

 the oesophagus with the result of diminishing the size of the 

 oesophagus to such a degree that at first only liquid nourish- 

 ment could be taken in, and finally the choice lay between 

 starvation and cessation of growth of brain. In Limulus we 

 already see the commencement of the struggle, and in the forma- 

 tion of the Scorpions and Arachnids we see not only how narrow 

 the oesophagus has become so that the whole class are able only to 

 take in liquid food — , and indeed in certain Arachnids a special 

 suction pump arrangement has been devised in their stomachs 



VOL. IX. pt. I. 3 



