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 Ammoccetes and 



LlMULUS. 



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 

 branchire, 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 lacunas, 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 



