302 CHORDATA. 
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 atrium. Its 
dorsal part lies above the atrium and communicates down 
the primary pharyngeal bars with the ventral part lying below 
the endostyle. 
‘The 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- 
merous 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 sudbcntestinal 
vein. The part running forwards from the liver is the 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 :— 
a Dorsal daria 
° 
TS _Branchial Veno a oe ub-infesfin 
i Ss al 
<—— <Live 
arlery vein 
Vascular, 
Blood- 
Vascular, 
The nervous system lies immediately dorsal to the 
notochord ; it consists of a long tube, the front 
portion of which forms a small drain and the 
rest the spinal cord. 
Nervous. 
