184 THE ORIGIN OF VERTEBRATES 
its own cartilage, its own set of visceral muscles, its own sense-organs, just as 
do the respiratory appendages of Limulus. 
The branchial unit in the vertebrate is not the gill-pouch, but the branchial 
bar or appendage between the pouches. Embryology shows how each such 
appendage grows inwards, how a celomic cavity is formed in it, similarly to the 
ingrowing of the branchial appendage in scorpions. 
We do not know how the paleostracan sea-scorpions breathed ; they resemble 
the scorpion of the present day somewhat in form, but they are in many respects 
closely allied to Limulus. The present-day scorpion is a land animal, and the 
muscles by which he breathes are dorso-ventral somatic muscles, while those of 
Limulus are the appendage-muscles. 
The old sea-scorpions very probably used their appendage-muscles after the 
Limulus fashion, being water-breathers, even although their respiratory appen- 
dages were no longer free but sunk in below the surface of the body. The 
probability that such was the case is increased after consideration of the method 
of breathing in Ammoceetes, for the respiratory muscles of the latter animal are 
directly comparable with the muscles of the respiratory appendages of Limulus, 
and are not somatic. Even the gills themselves of Ammoccetes are built up in 
the same fashion as are those of Limulus and the scorpions. The conception of 
the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately 
explains the formation of the vertebrate heart, which is so strikingly different 
from that of all invertebrate hearts, in that it originates as a branchial and 
not as a systemic heart, and is formed by the coalescence of two longitudinal 
veins. 
The origin of these two longitudinal veins is immediately apparent if the 
vertebrate arose from a paleostracan, for in Limulus and the whole scorpion 
tribe, in which the heart is a systemic heart, the branchiew are supplied with 
blood from two large longitudinal venous sinuses, situated on each side of the 
middle line of the animal in an exactly corresponding position to that of the two 
longitudinal veins, which come together to form the heart and ventral aorta of 
the vertebrate. The consideration of the respiratory apparatus and of its blood- 
supply in the vertebrate still further points to the origin of vertebrates from the 
Paleostraca. 
