FUNDAMENTAL FOEMS. 



59 



studying tlie relations of the axes to one anotlier, as explained 

 above (cf. Fig. 17). The importance of the month to the organism 

 causes the differen- 

 tiations which obtain 

 around it to have a 

 special value. These 

 differentiations are de- 

 veloped as tentacles 

 of various form^ and 

 cause the parts around 

 the mouth to be much 

 more varied in cha- 

 racter than those at 

 the aboral pole. 



If the body grows 

 in the direction of its 

 primary axis^ without 

 becoming attached to 

 the ground, the axes 

 may acquire modified 

 importance if locomo- 

 tion in the direction of 

 the animal's length be 

 established. The pri- 

 mary axis will remain 

 as before, but the 

 secondary axes will 

 necessarily differ ac- 

 cording to the signifi- 

 cance of the surfaces 

 which they connect. 

 When one and the same 

 surface always touches 

 the supporting object, 

 it becomes the ventral 

 surface, and the oppo- 

 site one becomes the dorsal. These two surfaces, the dorsal and 

 the ventral, ai^e placed under different conditions, and must therefore 

 be differentiated in different ways, while the two sides, or — when 

 the body is perfectly flattened out — the two lateral edges necessarily 

 come to differ in character from the dorsal and ventral surfaces. 



Such cases are instances of the development of only two 

 inequivalent secondary axes. One connects the ventral and dorsal 

 surfaces, and is the dorso-ventral axis (Fig. 18, a h), the other 

 connects the sides {c d) of the body, and is the transverse axis. The 

 surfaces which correspond to the poles of the first or dorso-ventral 

 axis are not, while those which corresjDond to the poles of the 

 transverse axis are, equivalent. A primitive condition which has 

 disappeared in the dorso-ventral axis in consequence of the differen- 



Fig. 17. Radiate fun- 

 damental form ; letters 

 as in Fig. 16. The an- 

 terior surface of the 

 body is seen in the lower 

 figure, and shows the 

 appendages (tentacles) 

 which are differentiated 

 along the two transverse 



Fig. 18. Diagram to show 

 the differentiation of the 

 secondary axes. In the 

 upper figure a cephalic 

 portion is indicated by 

 the development of a pair 

 of dorsal tentacles. The 

 lower is a transverse sec- 

 tion of the upper figure, 

 and the secondary axes 

 are consequently seen in 

 it. 



