Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FUNDAMENTAL FOE MS. 59 studying the relations of the axes to one another, as explained above (cf. Fig. 17). The importance of the mouth 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
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FUNDAMENTAL FOE MS. 59 studying the relations of the axes to one another, as explained above (cf. Fig. 17). The importance of the mouth 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 sig'nifi- 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, are 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 b), 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 correspond to the potas of the transverse axis are, equivalent. A primitive condition which has disappeared in the dorso-ventral axis in consequence of the differen-
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