326 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



The foot is thus attached to the lorica in such a way that it can bend to the right, 

 but not to the left. 



6. Toes. — The toes form perhaps the most peculiar characteristic of the Rat- 

 tulidse. Most of the Rotifera have two short posterior appendages attached to the 

 foot, placed side by side, and, like most paired organs, similar in form and size. But 

 in the Rattulidse we find the two toes in the majority of cases unequal, sometimes 

 strongly so, and not side by side. In some species one of the toes has almost disap- 

 peared, while the other has become immensely developed, forming a straight rod as 

 long as the body (Fig. E, 6-12). 



The steps in the series of changes by which this is brought about may be clearly 

 followed by comparing the toes of different species. In a few species (Diurella tigris 

 Muller, Diurella rousseleti Voight, etc.) the two toes are still equal, as in other 

 rotifers. One of these will serve best as a starting-point. We will select Diurella 

 tigris Muller, whose toes are shown in Figure G, 1. The toes form two long, curved, 

 pointed spine-like rods, of equal size. At the base of each are four small, flattened 

 spines (so-called substyles), which usually lie closely applied to the toes. The two 

 toes in Diurella tigris Muller are not placed exactly side by side, as they are in most 

 rotifers, but they partake of the prevailing asymmetry of the animals. The attach- 

 ment of the toes to the foot is oblique, like that of the foot to the body, so that the 

 right toe lies at a higher level than the left. The arrangement will be best under- 

 stood if we conceive it to have been formed as follows. The toes, originally curved 

 downward, have been twisted at their attachment to the foot, so that the concavity 

 of the curve now faces to the right instead of down, and the right toe lies above the 

 left (Fig. E, 1). The toes and foot can therefore now bend only to the right — not 

 toward the ventral side, as in most other rotifers. 



Now the right or upper toe begins to degenerate, until in some species there is 

 hardly a trace of it to be detected, while the left or lower toe becomes further devel- 

 oped. The steps in this process are easily traceable, and are shown in Figure G. In 

 Diurella stylata Eyferth (2) and D. brachyura Gosse (3) the toes are almost equal, 

 but the left is a little longer. In D. porcellus Gosse (4) the difference is greater. In 

 D. insignis Herrick (5) and D. tenuior Gosse (6) the right toe is about half as long 

 as the left. In Rattulus gracilis Tessin (7) it is about one-third the length of the left. 

 The right toe now forms a small rudimentary spine, which has its tip bent beneath 

 the main or left toe, and lies against the latter. Rattulus lophoessus Gosse (8) shows 

 a further step; R. elongatus Gosse (9), R. longiseta Schrank (10), R. scipio Gosse (11), 

 R. carinatus Lamarck (12), R. mulitcrinis Kellicott (13), and R. pusillus Lauterborn 

 (14) still further ones in the reduction of the right toe and corresponding increase 



