302 CHORDA TA. 



The cirri and the side-walls of the pharynx between the 

 pharyngeal clefts are supported by skeletal rods or bars. 



The coelom is well developed, a perivisceral cavity ex- 

 tending round the intestine and forming a dorsal mesentery 

 behind the atriopore ; but forwards its relations 

 are obscured by the presence of the atrmm. Its 

 dorsal part lies above the atrium and communicates down 

 the primary pharyngeal bars with the ventral part lying below 

 the endostyle. 



'I'he blood system is not unlike that of Ascidia, A dorsal 

 aorta or artery extends throughout the body. In the 

 pharyngeal region it is paired and receives nu- 

 Vascular ^"^erous efferent branchials from the walls of the 

 pharynx. The ventral vessel is a vein and is 

 interrupted at the liver in which it breaks up into small 

 capillaries. The part behind the liver is the subintestinal 

 vein. The part running forwards from the liver is thQ portal 

 vein^ which runs to the pharynx, on the ventral surface of 

 which it is continued as the branchial artery^ giving off 

 paired afferent branchials. The afferent and efferent bran- 

 chials really form continuous aortic arches. There is no 

 heart but the bases of the afferent branchials are contractile. 

 The arrangement by which the venous blood is supplied 

 direct to -the liver instead of passing directly forwards is 

 called the Hepatic-portal ' system and is characteristic of 

 Vertebrata, 



It should be noted that, as there is no true heart, the terms " artery" 

 and "vein" are not morphologically accurate, but are applied to the 

 vessels which correspond in structure and function with those of the 

 higher Chordata. 



The course of the blood is as follows : — 



The nervous system lies immediately dorsal to the 

 ^ notochord ; it consists of a long tube, the front 



portion of which forms a small brain and the 

 rest the spinal cord. 



