A COMPARATIVE STUDY OF THE CIRCULATORY SYSTEM 255 



as intermediary agents between the distant cells and the nutritive 

 material in the alimentary passage. In the medusa well-marked 

 gastro vascular streams may be observed. The lower vermes ex- 

 hibit an arrangement very "similar to that found in the coelenterates. 

 In the slightly higher forms, however, the alimentary tract is com- 

 pletely separated from the general body cavity, so that the gas- 

 tric prolongations are enabled to assume the function of true cir- 

 culatory channels. The fluid within them is albuminous in character, 

 and is moved from place to place by differences in pressure produced 

 by the general movements of the body. In some annelids, the cir- 

 culatory system is fully differentiated and consists of a dorsal and a 

 ventral tube which are connected with one another 

 by several branches. As the latter, as well as the 

 adjoining segments of the dorsal tube, are auto- 

 matically active, these forms may be said to be in 

 possession of a real heart which, however, presents 

 a most rudimentary structure. Its most essential 

 characteristic is its tubular shape. In Arenicola, 

 the main cardiac cavity is constricted at one point 

 so that the cardiac tube as a whole appears as two 

 distinct compartments. 



Similar differences are to be noted among the 

 vertebrates. Amphioxus, for example, does not 

 possess a distinct heart, a portion of its posterior 

 aorta being equipped with automatic power. It 

 should be remembered, however, that this animal 

 presents the first indications of a portal circuit, be- 

 cause the dorsal aorta gives off certain branches to 

 the intestine, from which organ the blood is then 

 collected by a single tube, which is known as the 

 portal vein. Having traversed the capillaries of 

 the liver, the blood is eventually returned into the 

 ventral aorta. 



In the lower animals, the power of rhythmic activity extends over 

 relatively long segments of the dorsal and lateral blood-vessels; but 

 in the fishes the heart loses its diffuse tubular character, and the power 

 of contraction becomes restricted to a particular area of the vascular 

 system. These animals are in possession of a cardiac mechanism which 

 occupies the ventral extent of the body-cavity and presents a structure 

 very similar to that found in the higher animals. It is protected on 

 all sides by a membrane which is reflected from its base to form a pouch, 

 the so-called pericardial sac. The organ, as a whole, is composed of 

 two compartments, an antechamber or auricle, and a main chamber 

 or ventricle. Moreover, as the veins do not unite with the auricle as 

 separate tubes, but become confluent, a vestibular chamber is formed 

 which is commonly designated as the sinus venosus. Quite similarly, 

 the aorta does not arise from the ventricle itself , but from an appendage, 



FIG 124. DIA- 

 GRAM TO SHOW THE 



COURSE OF THE BLOOD 

 THROUGH THE FISH 

 HEART. 



SV, sinus venosus; 

 A, auricle; V, ven- 

 tricle; BA, bulbus ar- 

 teriosus; A, aorta 

 with (C) arteries to 

 gill plates. 



