328" ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



Connected with the toes are two sac-like mucous reservoirs, shown in Figure G, 

 1, 10, 15. These pour out upon the toes a tenacious substance by which the animals 

 may attach themselves to surrounding objects. Hand in hand with the reduction 

 in the right toe goes a reduction in the mucous reservoir which is connected with it. 

 The reduction of the reservoir is not so extensive as that of the toe, however, and 

 it never completely disappears. Stages in this reduction are shown in Figure G, 10, 

 15. 



7. Antennae — Of the other external structures in the Rattulidae, it is chiefly the 

 so-called antennae in which the asymmetry is evident. These are small projections 

 through the lorica, bearing at the tip a bunch of setae, supposedly sensory in charac- 

 ter. There are three of these antennae, one dorsal and two lateral. 



The dorsal antenna (Fig. E, at.d.) is found, in most rotifers; on the dorsal surface, 

 some distance back from the anterior edge of the head. The position of the dorsal 

 antenna probably indicates the dorsal median line, so that its position in the Rattulidae, 

 in which it is difficult to locate with certainty the median line, owing to the twisted 

 form, is of interest. It lies usually to the left of the oblique ridge above described, 

 usually at about the left edge of the striated area (Fig. E, 1, 8, 9). 



The lateral antennae are placed in most other free-swimming rotifers (as in the 

 Notommatidae, from which the Rattulidae are in all probability derived), on each 

 side, in the posterior third of the body, symmetrically with relation to each other. 

 Many species of Rattulidae have preserved nearly this primitive condition, though 

 usually with slight variations (Fig. E, 1, 3, 6, 9). But in some cases there is a very 

 remarkable asymmetry in the position of the two antennae. In Diurella stylata 

 Eyferth (Fig. E, 2) and in Rattulus cylindricus Imhof the left antenna lies in front 

 of the middle of the body, while the right one is far back, near the posterior end. 



8. Internal Organs — The internal organs partake to a considerable degree of the 

 asymmetry so characteristic of the external anatomy of the Rattulidae. I shall men- 

 tion here only those features in which the asymmetry is shown. 



The brain (Fig. E, 10, ence.) is frequently formed of several lobes, the left one 

 of which bears at its posterior end the eye. The mastax, or muscular pharynx, is 

 likewise frequently shown as an unsymmetrical form, this being a consequence of 

 the asymmetry of the trophi, to be described at once. 



The trophi, or chitinous jaws, usually show a considerable degree of asymmetry, 

 though there is much variation in this among the different species. Taking the trophi 

 of Rattulus longiseta Schrank (Fig. H, 1, 2) as a type, we may distinguish the following 

 parts. There are two lateral portions, known as mallei, and a central structure, the 

 incus. Each of these is composed of several portions. Each malleus consists of two 



