1899] ANIMAL SYMMETRY 57 



currents of the medium) whilst the two ends of the axis will have 

 dissimilar environments. This axis corresponds to the axis of 

 symmetry of the axo-symmetric animals which are sedentary. 



(2) Pelagic plankton. — If a perpendicular axis be drawn downwards 

 from the surface of the medium of notation, the environment on all 

 sides of this axis will be similar whereas its two ends will be very 

 dissimilar. Hence a floating (drifting) organism will have an axis 

 parallel to the above and with similar properties. According to the 

 principles of correlation, we find that the drifting plankton 

 (Medusae, Arcella, Pyrosoma, etc.) are axo-symmetric about an axis 

 which is at right angles to the surface. It may be noted that move- 

 ment along this axis, i.e. the axis of symmetry, does not in any way 

 affect the conditions of symmetry (Medusae, Ctenophora). It is quite 

 irrelevant to the argument that Medusae may be descended from 

 sedentary forms. They are axo-symmetric and pelagic drifters, 

 agreeing thereby with an enormous number of other forms. 



(3) Free-swimming with axial rotation. — If an organism move in 

 one definite direction there is an evident dissimilarity in environmental 

 conditions between the two terminal points of the axis of direction. If 

 this direction correspond with the axis perpendicular to the surface it does 

 not affect the axo-symmetry, for it only accentuates a dissimilarity which 

 already existed, but if it be at any angle to the perpendicular there will be 

 produced two axes of dissimilarity, and unless other factors intervene an 

 axo-symmetric organism will be out of harmony with such conditions. 

 If, however, the organism rotates about its axis of locomotion, the dis- 

 similarity due to the physical conditions will be nullified, and the parts 

 on all sides of the axis of direction will, in response to the similar 

 environment, tend to become symmetrical, so that the axis of locomotive 

 direction will be converted into the axis of symmetry and the organism 

 will become axially symmetric. 



Those who have observed the locomotion of gastrula-larvae speak 

 of a spiral motion due to a movement forward along the main axis, 

 combined with a rotation about the same. It is also quite possible 

 that in many of the more active members of the pelagic plankton, in 

 which the locomotion often brings the axis of symmetry at an angle 

 with the perpendicular to the surface, a free capacity for axial rotation 

 tends to perpetuate the axial symmetry. 



In brief, the axo-symmetric organisms occur principally in the 

 Protozoa, Porifera, Coelentera, and Echinoderma, and they are typically 

 pelagic drifters, sedentary forms, or free-swimmers which rotate about 

 the axis of locomotion, all of which habitats correspond to the 

 theoretical conditions. In a great number of the axo-symmetric 

 Coelentera there are two phases in the life -history (medusoid and 

 hydroid) corresponding to the two principal conditions of existence in 

 this type of symmetry, i.e. drifting and sedentary. 



3. Piano -symmetry (Heteraxonia of some morphologists). — This 



