204 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



can loosen its hold with the suckers voluntarily; and, by 

 loosening those at one end of the body, swinging this end 

 laterally and refastening it, and then loosening the other 

 end of the body and swinging and refastening it, a slow but 

 safe locomotion, chiefly lateral, is possible. The larvae 

 move about not a little, especially from the necessity of 

 continually moving from the edge out farther into the stream 

 as the water of the little stream gradually lessens in quantity. 

 The structure of these suckers and the manner of their 

 working are of interest. The ventral (external) aspect 

 of a sucker (PI. XXI, fig. 5) shows a central opening, 

 surrounded by a strong, flexible, concave rim, marked 

 with alternating concentric thicker and darker and thin- 

 ner and lighter bands of chitin. The rim projects con- 

 siderably ventrad, so that a considerable free or air space 

 is enclosed by the rim when its outer edge is applied 

 to any surface. In dorso-ventral sections transversal to 

 the body of the larva (PI. XXI, fig. 6), the whole structure 

 of the sucker is apparent. The cup-shaped sucker is seen, 

 after all, not to consist of a rim around a circular opening, 

 but to be simply a part of the outer body-wall (true skin 

 and chitin cuticula) peculiarly folded and modified to act as 

 a sucker. The projecting cup-like part of the sucker is 

 coated with chitin, so as to be thick and strong, although 

 still flexible. At its base the skin is almost free from chitin, 

 thrown into fine folds, and bent in toward the interior of 

 the body and then out again. Here it is greatly thickened 

 by a circular, lens-shaped deposit of chitin, which is slightly 

 larger than the inner neck of the sucker, which it closes 

 internally. The neck of the sucker is the apparent central 

 opening, and the lens-shaped thickening is the bell-shaped 

 structure, which closes this opening internally, as seen in 

 looking at the sucker from the under or external side. The 

 structure of the sucker is all plainly shown in PI. XXI, 

 fig. 6, and can be much more readily understood from an 

 inspection of the figure than from reading this description. 

 Attached to the inner face of the lens-shaped "stopper" of 

 the sucker are two great muscles which run dorsally and 



