IS 



JEAN DAWSON. 



the ectal valve is attached to the external end and on the cephalic 

 margin. When the free border of the ectal valve is tightly 

 stretched in the cartilaginous ring (Fig. 12) the ental valves, if 

 forced outward by pressure within the gill sac, strike against the 

 ectal valve and close the external branchiopore thus preventing 

 the escape of water. 



If the border of the ectal valve is relaxed it does not afford a 

 support for the ental valves which are then forced out past it by 

 the pressure of the water and thus permit water to pass out 

 through the external branchiopore (Fig. 10). Thus while the 

 valves of the external branchiopore can at no time prevent the 

 entrance of water into the gill sac, they are able under certain con- 

 ditions, to prevent its exit. 



The stretching of the ectal valve is effected by the action of two 

 muscles which elongate the dorso-ventral axis of the cartilaginous 

 ring to which the valve is attached. The first one is seen when 

 the integument is removed and lies on the cranial side of the gill 

 cleft. This small band of muscles lies loosely around the 

 cephalic side of the gill opening, and is attached to the longi- 

 tudinal bars immediately dorsal and ventral to the vertical axis 

 of the opening and to the ring of cartilage (Fig. 13). This may 



FlG. 12. The cartilaginous ring of the external branchiopore is oval in shape thus 

 stretching the ectal valve taut, a, ectal valve ; </, buccal fringe ; e, point on the car- 

 tilaginous ring ; r, cartilaginous ring. Figs. II and 12, placed back to back as here 

 printed, form together a model of a left external branchiopore with closed valves. 



be called the ectal muscle. 



