336 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



IV. SUMMARY. 



In addition to the usually recognized general types of structure — the radially 

 symmetrical, characteristic of a fixed life; the bilaterally symmetrical, characteristic 

 of animals which keep dorsal and ventral surfaces in different relations with the 

 environment — we may distinguish a third type of structure, equally characteristic of 

 a certain method of life and movement. This is an unsymmetrical or spiral type 

 of structure; it is characteristic of animals which swim in spirals, and is to be con- 

 sidered an adaptation to the spiral course. 



The spiral course is the simplest device for permitting an organism to make 

 progress in a given direction through the free water, without having the parts of the 

 body elaborately adjusted so as to balance each other accurately. Not having such 

 elaborate adjustment, small organisms would swim in circles, were it not for their 

 revolution on the long axis of the body. This converts the circle into a spiral course, 

 permitting progress to be made. 



In such a spiral course the organism maintains its body in a definite relation 

 to the axis of the spiral, the same surface always facing outward, the opposite surface 

 facing the axis of the spiral. 



Many organisms which swim in this manner have the body structurally adapted 

 to this movement, the form approximating in some degree to a segment of a spiral. 

 An unsymmetrical structure results. 



Such unsymmetrical structure, adapted to a spiral path, is found in most of the 

 free-swimming flagellate and ciliate Infusoria. 



This unsymmetrical form is somewhat modified in some of the Ciliata. In the 

 Hypotricha we have a group in which, owing to the habit of creeping along surfaces, 

 bilateral symmetry is becoming superposed upon the primitively spiral form, though 

 the process has not gone far. In the Peritricha, with their fixed method of life, 

 radial symmetry is becoming superposed on the primitively spiral form, though the 

 latter is still very evident. 



In the Rattulidse we have an analogous case, where in a group of primitively 

 bilateral organisms (Rotifera) the spiral form, as an adaptation to spiral movement, 

 is becoming superposed on the originally bilateral form. The animal swims in a spiral, 

 of which its twisted body forms a segment. 



The twisted form, the oblique ridge, the teeth on the right side at the anterior 

 dorsal edge of the lorica, the oblique position of the foot and toes, and most of the 

 other unsymmetrical features of many species of the Rattulidse become intelligible 



