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 ccelomic cavity is formed in it, similarly to the 

 ingrowing of the branchial appendage in scorpions. 



We do not know how the palteostracan 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 aj>pendage-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 Ammoccetes, 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 long-itudinal 

 veins. 



The origin of these two longitudinal veins is immediately apparent if the 

 vertebrate arose from a palaeostracan, for in Limulus and the whole scorpion 

 tribe, in which the heart is a systemic heart, the branchife 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 

 Palasostraca. 



