THE EVIDENCE OF THE RESPIRATORY APPARATUS 175 
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 gy. 69.—Comparison or Brancntan La- 
the corresponding positions of MELL& OF LIMULUS AND SCORPIO WITH 
the central nervous system, is BRaNCHIAL LAMELLEZ OF AMMOCETES. 
one of the principal reasons "sis: B, Deansial pean 
why all the advocates of the moceetes (after Nestler). 
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, ¢.¢. 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 
