sect, xii PHYLUM CHORDATA 377 



anterior wall only. On the anterior wall of the spiracle is a 

 rudimentary gill, the pseudo-branch or spiracular gill, in 

 the form of a few slight ridges. 



The heart is situated in the pericardial cavity, on the ventral 

 aspect of the body, in front of the pectoral arch and between 

 the two series of branchial pouches. The heart consists of 

 four chambers, — sinus venosus {s. z<), auricle (au), ventri- 

 cle (v), and conus arteriosus (c. art), through which the 

 blood passes in the order given. The sinus venosus is a thin- 

 walled, transverse, tubular chamber, into the ends of which 

 the great veins open. It opens into the auricle by an aper- 

 ture, the sinu-auricular aperture. The auricle is a large, 

 triangular, thin-walled chamber, situated in front of the sinus 

 venosus and dorsal to the ventricle. Its apex is directed 

 forwards, and its lateral angles project at the sides of the 

 ventricle ; it communicates with the ventricle by a slit-like 

 aperture guarded by a two-lipped valve. The vetitricle is a 

 thick-walled, globular chamber, forming the most conspicu- 

 ous part of the heart when looked at from the ventral sur- 

 face. From it the conns arteriosus runs forwards as a 

 medium stout tube to the anterior end of the pericardial 

 cavity, where it gives off the ventral aorta. It contains two 

 transverse rows of valves, anterior and posterior, the former 

 consisting of three, the latter of three or four. The ventral 

 aorta (Fig. 227, v. ao) gives origin to a series of paired affer- 

 ent branchial arteries (a. br. a), one for each branchial 

 pouch. 



From the gills the blood passes by means of the efferent 

 branchial arteries (e.br.d). These efferent vessels form a 

 series of loops, one running around the margin of each of 

 the first four internal branchial clefts : a single vessel runs 

 along the interior border of the fifth branchial cleft and 

 opens into the fourth loop. The four main efferent bran- 



