XIII 



PHYLUM CHORDATA 



259 



consists, as usual, of a visceral layer closely adherent to the heart, 

 and a loose parietal layer, the two being continuous at the bases of 

 the great vessels and separated by a small quantity of pericardial 

 fluid. 



The heart consists of a sinus venosus (Figs. 876 and 880, s. v.), 

 right and left auricles (r. au., I. au.), a ventricle (v., 0.),and a conus 

 arteriosus (c. art.). As in Dipnoi, the sinus venosus opens into the 

 right auricle, the pulmonary veins into the left ; a striking advance 



FIG. 878. Rana temporaria. The heart from the ventral aspect with the cavities laid open. 

 a, a', bristle in left carotid trunk ; au. v. v. auriculo-ventricular valves ; b. b'. bristle in left 

 systemic trunk ; c, c', bristle in left pulnio-cutaneous trunk ; car. a. carotid artery ; car. gl. 

 carotid plexus ; c. art. conus arteriosus ; car. tr. carotid trunk ; 1. au. left auricle ; Ig. a. lingual 

 artery ; 1. v. longitudinal valve ; pul. cu. tr. pulnio-cutaneous trunk ; pul. v. aperture of pul- 

 monary veins; /. au. right auricle; s. au.-ap. sinu-auricular aperture; spt. aur. septum 

 auricularum ; v. v'. valves ; vt. ventricle. 



is seen in the greatly increased size of the left 'auricle and its 

 separation by a complete partition, the septum auricularum (Fig. 878, 

 spt. aur.), from the right. The two auricles open,, by a common 

 auriculo-ventricular aperture, guarded by a pair of valves (au. v. v.), 

 into the single ventricle. Thexonus springs from the right side of 

 the base of the ventricle ; it is separated from the latter by three 

 small semilunar valves (v.}, and is traversed obliquely along its whole 

 length by a large flap-like longitudinal valve (I. v.) which springs 

 from its dorsal wall and is free ventrally. The conus passes without 

 change of diameter into a bidbus aortce, the two being separated by 



s 2 



