14 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



aided by a valve device, to be described in the next paragraph. The 

 atrial sphincters are for the most part very delicate and able to per- 

 form little labor, but their function is merely to close the atrial aper- 

 ture during diastole of the body. The enlarging of the body after 

 contraction is due to the elasticity of the test, which in this species is 

 very delicate. When once the atrial pore is closed, the suction due 

 to the enlarging of the body will tend to keep it closed. Very slightly 

 developed sphincters therefore are enough. It is important to have a 

 series of sphincters instead of a single one at the edge of the atrial 

 siphon. If only the edge of the aperture closed, leaving the rest of 

 the siphon widely open, the tip would be introverted, and then even 

 a strong sphincter at the edge of the aperture could hardly prevent 

 water entering the body through the atrial pore. But the narrowing 

 or complete closing of the whole atrial siphon avoids any danger of 

 such introversion. The heavier sphincter muscle at the base of the 

 atrial siphon probably aids in preventing the siphon as a whole from 

 becoming introverted. 



Reference was made in the previous paragraph to the valve action 

 of the lower lip. The structural conditions bringing about this 

 effect are shown in figure 6, plate 2, a dorso-lateral-anterior view of 

 the mouth region of the solitary Cyclosalpa pinnata. The lower lip 

 is turned inward, forming a deep rounded pouch. The edge of the 

 lip is a thin irregular flap. At the base of the flap are the two admar- 

 ginal sphincter muscles. Further ventral is the third sphincter, 

 which serves to support the bulging middle portion of the pouch, 

 while the admarginal sphincters cause a stronger contraction of the 

 inturned portion of the lip, giving the pouch form to the whole lower 

 lip. The backward pull of the dorsal oral retractor goes directly to 

 the inturned portion of the lower lip, so that this is very strongly 

 inturned. The contraction of the dorsal lip muscles, of the dorsal 

 retractor system, brings the upper lip down into contact with the 

 lower lip. The thin flap at the inturned edge of the lower lip lies 

 against the inside of the contracted upper lip, serving effectively to 

 prevent any egress of the water. The ventral retractor, with its two 

 dorsal sphincters and one ventral sphincter, strengthen and support 

 both lips. The actual closing effect, however, is due chiefly to the 

 dorsal retractor system. 



The positions of the gill, dorsal tubercle, endostyle, and peripharyn- 

 geal bands are sufficiently indicated in figure 1. The form of the 

 aperture of the dorsal tubercle is shown in figure 2. 



The alimentary canal consists of the huge pharynx (imperfectly 

 demarcated by the gill from the atrium), the esophagus, intestine, 

 and two caeca. No well-marked stomach is found. The flaring 

 trumpet-shaped esophageal aperture lies at the ventral end of the 



