306 



COMPARATIVE ANATOMY 



muscular conus arteriosus (Fig. 246, a) : these are^most numerous 

 in Elasmobranclis and Ganoids. There is a tendency, however, for 

 the posterior valves, or those which lie nearest the ventricle 



D.ca. 



fSix.s. 



tra^a. 



Fig. 245. — Heart or A, Zyycena malleus, fkom the ventral side ; B, of Acan- 

 thias ruli/aris, from the dorsal side, with the atrium cut open (after 

 "Rose) ; C, OF A Teleost (Silurus glanis). 



A, A, atria; a, a, auricular appendages ; V, ventricle ; tr (in B) and Ba (in C), 



bulbus arteriosus ; Ir (in A) and co (in B), conus arteriosus, tr (in C), ventral 



aorta. 

 D.C.d and D. C. s, right and left precavals ; V.a.d. and T'. a. s, right and left 



valve of the sinus venosus ; O.a.v, atrio- ventricular aperture ; l,o — 4,(X, 



alferent branchial arteries. 



gradually to undergo reduction (b). The most anterior row always 

 persists, and corresponds to the single row of valves between the 

 ventricle and bulbus in Teleosts (c) and Cyclostomes. Together 

 with the reduction of these valves, the conus arteriosus also be- 

 comes reduced in the last-mentioned forms, so that the non- 



