174 THE ORIGIN OF VERTEBRATES 
great interest, but their consideration may also better be deferred to 
the chapter dealing with those special sense-systems known as the 
lateral line and auditory systems. 
COMPARISON OF THE BRANCHIAL CIRCULATION IN AMMOC(ETES AND 
LIMULUS. 
Closely bound up with the respiratory system is the nature of 
the circulation of blood through the gills. Before, therefore, proceeding 
to the consideration of the segments in front of those which carry 
branchie, it is worth while to compare the circulation of the blood 
in the gills of Limulus and of Ammoccetes respectively. 
In all the higher vertebrates the blood circulates in a closed 
system of capillaries, which unite the arterial with the venous systems. 
In all the higher invertebrates this capillary system can hardly be 
said to exist ; the blood is pumped from the arterial system into blood, 
spaces or lacune, and thus comes into immediate contact with the 
tissues. From these it is collected into veins, and so returned to the 
heart. There is, in fact, no separate lymph-system in the higher 
invertebrates ; the blood-system and lymph-system are not yet 
differentiated from each other. This also is the case in Ammoccetes ; 
here, too, in many places the blood is poured into a lacunar space, 
and collected thence by the venous system; a capillary system is 
only in its commencement and a lymph-system does not yet exist. 
In this part of its vascular system Ammoccetes again resembles the 
higher invertebrates more than the higher vertebrates. 
This resemblance is still more striking when the circulation 
in the respiratory organs of the two animals is compared. A 
branchial appendage is essentially an appendage whose vascular 
system is arranged for the special purpose of aerating blood. In the 
higher vertebrates such a purpose is attained by the pulmonary 
capillaries, in Limulus by the division of the posterior surface of the 
basal part of the appendage into thin lamellar plates, the interior of 
each of which is filled with blood. The two surfaces of each lamella 
are kept parallel to each other by means of fibrous or cellular strands 
forming little pillars at intervals, called by Macleod “colonettes.” 
A precisely similar arrangement is found in the scorpion gill-lamella, 
as seen in Fig. 69, A, taken from Macleod. In Ammoccetes there are 
no well-defined branchial capillaries, but the blood circulates, as in 
