The Microscope. 5 



is exceedingly curious and interesting to see with what facihty they 

 use the caudal appendages, sticking them to the glass slide or cover 

 in such a manner that, by careful focussing, one can see the sucker- 

 like action of the tips of these organs while they sway about one 

 way and the other in the water." The ducts seem to open in the 

 center of the enlarged ends of the caudal prolongations. 



In all the species with dorsal scales or setose appendages, these 

 commonly extend upon the ventral surface as far as the outer 

 margin of each ciliary band. The latter are, as a rule, two only. 

 In but one form, Ch. larus, are there four, and even with it they 

 are, according to my observation, as often two as four. The bands 

 are near the lateral borders of the flattened ventral surface, and 

 extend from near the mouth to the caudal furcation, and subserve 

 locomotion onl}-. The space between these ciliary lines is, in most 

 species, entirely smooth and naked. In some, however, it is hispid 

 with setose hairs, or clothed with short, recurved prickles. In still 

 others these additions are represented by a few long setse near the 

 posterior bifurcation. 



The mouth has a more complicated structure than appears at 

 first glance. It is surrounded by a smooth ring, which may be 

 called the oral annulus, somewhat elevated above the general surface, 

 and additionally encircled by a series of setose, non-vibratile cilia. 

 The oral annulus is so deeply striate vertically that in a direct ventral 

 view it seems to be marginally beaded, and from the interspaces 

 between these beads, or from the vertical furrows, the oral setse 

 appear to take their origin. These hairs are visible in every species 

 that has come to my notice. The beads of the oral annulus are in 

 some forms very minute, and in others are entirely suppressed. The 

 cilia are in all the cause of an interesting optical illusion. That 

 they project beyond the oral annulus more or less at right angles to 

 the ventral plane can be positively determined only when the animal 

 is viewed in profile. Then they are seen as conspicuous projections 

 (Plate II, figures 21, 22 and 35), the animal seeming to be able to 

 control their position so far, at least, as approximation and separa- 

 tion of their distal extremities are concerned. In the egg, before 

 the complete developement of the embryo, these cilia present a fas- 

 cicled aspect similar to that shown in Plate II, figure 35, a position 

 also not rare in the mature, free-swimming forms. But when the 

 animal is examined with the ventral surface upward, the oral annu- 

 lus seems to be closed by a convex membrane, pierced by a small 

 central aperture, and strongly striated. This false appearance is 



