THE EVIDENCE OE THE RESPIRATORY APPARATUS I 75 



the invertebrate gill, in a lamellar space ; here, also, as Nestler has 

 shown, the opposing walls of the gill-lamella are held in position by 

 little pillar-like cells, as seen in Fig. 69, B, taken from his paper. 



In this representative of the earliest vertebrates the method of 

 manufacturing an efficient gill out of a lacunar blood-space is pre- 

 cisely the same as that which existed in Limulus and the scorpion, 

 and, therefore, as that which existed in the dominant invertebrate 

 group at the time when vertebrates first appeared. This similarity 

 indicates a close resemblance between the circulatory systems of the 

 two groups of animals, and therefore, to the superficial inquirer, would 

 indicate an homology between the heart of the vertebrate and the 

 heart of the higher inverte- 

 brate ; but the former is situ- 

 ated ventrally to the gut and 

 the nervous system, while the 

 latter is composed of a long 

 vessel which lies in the mid- 

 dorsal line immediately under 

 the external dorsal covering. 

 Indeed, this ventral position of 

 the heart in the one group of 

 animals and its dorsal position 

 in the other, combined with 

 the corresponding positions of 

 the central nervous system, is 

 one of the principal reasons 

 why all the advocates of the 

 origin of vertebrates from the 



Appendiculata, with the single exception of myself, feel compelled to 

 reverse the dorsal and ventral surfaces in deriving the vertebrate 

 from the invertebrate. But there is one most important fact which 

 ought to make us hesitate before accepting the homology of the 

 dorsal heart of the arthropod with the ventral heart of the vertebrate 

 —The heart in all invertebrates is a systemic heart, i.e. drives the 

 arterial blood to the different organs of the body, and then the veins 

 carry it back to the respiratory organ, from whence it passes to the 



heart. 



The only exception to this scheme is found in the vertebrate 

 where the heart is essentially a branchial heart, the blood being 



Fig. 69.— Comparison of Branchial La- 

 mellae of Limulus and Scorpio with 

 Branchial Lamellae of Ammoccetes. 



A, Branchial lamellae of Scorpio (after 

 Macleod) ; B, Branchial lamellae of Am- 

 moccetes (after Nestler). 



