14 



BULLETIN OF THE BUREAU OF FISHERIES. 



air tubes or tracheae, by means of which the larva breathes. The anterior and 

 posterior margins of this groove form a pair of Hps, which can be opened or closed 

 at will and thus control the breathing. The posterior lip extends diagonally up- 

 ward and backward to the level of the dorsal surface of 

 the last abdominal segment, but though it thus appears 

 in a dorsal view it really belongs to the ventral surface. 

 At its center is a narrow chitinous flap extending from 

 the margin of the lip back along the outer surface (fig. 

 bjji) . The lateral edges of the flap are turned forward, 

 giving it the shape of the letter U, and it forms the 

 posterior or ventral cover of the anal opening. Inside 

 of this flap the end of the intestine {i) projects a little 

 from the floor of the groove and the anus (a) opens 

 halfway from the floor to the edge of the lower lip. In 

 front of the anus is a fingerlike papiUa on either side, 

 tipped with a long bristle. When the lips of the groove 

 are closed, these papillse (p) come together in front of 

 the anus and form an anterior or dorsal cover for the 

 anal opening. In this way the anus can operate when 

 the lips are closed, and whatever passes out of it is 

 kept away from the tracheal openings. The central 

 portion of the anterior lip is also somewhat chitinized 

 over a trapezoidal area extending from the edge of the 

 lip forward along the dorsal surface the full length of 

 the last segment. 



On either side of the intestine, in the deepest por- 

 tion of the groove, is the opening of the tracheal air tube {t), which is elliptical 

 in outline, with the long diameter inclined at an angle of 45° to the body axis. 

 Each opening is covered with a membraneous valve 

 slit along the outer margin of the ellipse, which effec- 

 tually closes the opening to the entrance of water. 



On the ventral surface of the last abdominal seg- 

 ment are the cerci, two slender cylinders from 3 to 4 

 mm. in length in a full-grown larva and about 0.5 mm. 

 in diameter. They are more or less wrinkled trans- 

 versely and normally are naked, but often become cov- 

 ered with growths of algge and Protozoa. There is no 

 central lumen in these cerci, but the entire space is 

 filled with tissue and contains many muscles. When 

 the larva rises to the surface of the water to breathe, 

 the cerci naturally flatten themselves along the surface 

 film at right angles to the longitudinal body axis. This 

 not only furnishes the larva with a secure hold upon the surface film, but it also 

 opens the lips of the transverse groove and holds them open as long as the larva 

 remains at the surface. When the hold upon the surface film is released, the cerci 

 trail backward in the water and close the transverse groove. In front of the 



Fig. 5. — Transverse groove on dorsal sur- 

 face of last segment of abdomen, a, 

 anus; c, cerci; ^, chitin flap; i, intes- 

 tine; p, papillae; t, openings of tracheal 

 air tubes. 



Fig. 6.— Side view of last segment of 

 abdomen, showing depth of dorsal 

 groove and the papilla on the ventral 

 surface. 



