INTERNAL ANATOMY. 47 



of the Arthropoda (Insects and Crustacea) and the 

 Mollusca are characterised by the possession of a definite 

 muscular heart, yet in the various groups of worms there 

 are many which possess a very elaborate vascular system, 

 while not one of them possesses a heart. In fact, in the 

 last-mentioned forms, the place of a heart is taken, func- 

 tionally, by contractile blood-vessels. And this is the case 

 with Amphioxus. Among the Vertebrates, including 

 the Ascidians, it forms the unique instance in which such 

 an acardiac condition of the vascular system is met with. 



Lying below the pharynx in the endostylar ccelom, there 

 is a blood-vessel known as the bmncJiial artery, which con- 

 tracts more or less rhythmically, and corresponds in its 

 position and relations to the heart and truncus arteriosus 

 of the higher forms. 



Fig. 20. Diagram illustrating the chief parts of the vascular system of 

 Amphioxus. (Constructed after J. MULLER and SCHNEIDER.) 



The arrows indicate the direction of flow of the blood, ch. Notochord. hep. 

 Hepatic ccecum. af. Afferent branchial vessels (vascular bulbils of J. Miiller) 

 entering the primary bars from br.a, the branchial artery ; the efferent branchial 

 vessels are seen emerging from the tops of both primary and secondary bars and 

 running into d.a, the dorsal aorta. From the dorsal aorta, the blood enters the 

 capillaries over the wall of the intestine (indicated by the dark reticular shading), 

 and finally reaches s.i.v, the sub-intestinal vein. The latter carries the blood to the 

 base of the hepatic ccecum, over which it passes into another system of capillaries 

 (not indicated), and is then collected into A.v, the hepatic vein, which passes back- 

 wards and curves round into the branchial artery. 



From this branchial artery, lateral branches running up 

 into the primary bars of the pharynx are given off on both 

 sides alternately. (Cf. Fig. 20.) There appears to be no 



