322 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



Colpidium, Loxodes, Spirostomum, always turn, when stimulated in almost any way, 

 toward the aboral side; all the Hypotricha, toward the right side (which is perhaps 

 equivalent to the aboral side). This is especially striking when localized stimuli 

 are used. If with a fine glass rod we tap Stylonychia on the left side, it turns toward 

 the right (hence away from the source of stimulus) ; if we tap it on the right side, it 

 still turns toward the right side (hence in this case toward the source of stimulus). 

 The reaction is the same whatever the nature of the stimulus; heat, cold, chemicals 

 of any sort, mechanical shock, an electric shock, or a sudden change in the direction 

 of an electric current passed through the animal — all produce in Stylonychia the 

 same reaction, a turning toward the right side. This method of reaction is charac- 

 teristic for many, perhaps most, Infusoria, both Flagellata and CUiata. 



It is thus evident that in these organisms the characteristic unsymmetrical struc- 

 ture is closely correlated with the method of life and behavior. Among the Ciliata 

 we find two groups of organisms which show interesting examples of a tendency 

 toward a modification of this characteristically unsymmetrical form. The Hypotricha 

 are a group in which, owing to their habit of creeping along surfaces, dorsal and 

 ventral sides have been developed; in other words, bilateral symmetry is becoming 

 superposed on the primitively spiral form, though the process has not gone far. In 

 the Peritricha, with their fixed method of life, radial symmetry is becoming super- 

 posed on the primitive spiral form, though the latter is still very evident. 



III. ROTIFERA. 



Passing now to the Rotifera, we find in these, in contradistinction to the 

 Infusoria, a group of organisms which are primitively and typically bilaterally 

 symmetrical. But among these bilateral organisms we find a family, the Rattulidae, 

 which are almost throughout unsymmetrical in structure. We wish to inquire into 

 the significance of this asymmetry. 



No thorough account of the structure of these animals has ever been given in 

 such a way as to bring out clearly the nature of the unsymmetrical characteristics. 

 Such an account I shall attempt to give in the following pages, and this will be 

 concluded by a discussion of the biological significance of the asymmetry. 



The Rattulidae are a small family of free-swimming Rotifera, containing all 

 together about forty to forty-five species. They are found as a rule among aquatic 

 plants, in the quiet parts of lakes and streams. In such regions they are often among 

 the most abundant of the Rotifera. 



