THE SHAPES AND SIZES OF ANIMALS 133 



evidence. Much greater difficulty is offered by 

 the Ctenophora, in which biradiate symmetry is 

 usually well established, and by some Medusae, in 

 which the number of tentacles arising from the 

 margin of the bell may be reduced to two, or even 

 to a single one ; in these latter cases however 

 the biradiate symmetry is probably independently 

 acquired, as theoretically it might readily be by 

 any radiate animal. 



We have next to consider the type of animal 

 shape spoken of as bilaterally symmetrical, which 

 must be carefully distinguished from the biradiate 

 symmetry we have just been describing. In 

 biradiate symmetry, as in a sea anemone, there is 

 one divisional plane or plane of symmetry, by 

 which the animal can be divided into identical 

 halves ; this plane however concerns the internal 

 organs only, and has no constant relation to the 

 movements of the animal. In cases of bilateral 

 symmetry on the other hand, the animal, as in a 

 worm, a lobster, or a frog, is divided by a median 

 vertical plane into symmetrical right and left halves, 

 while furthermore a distinction may be readily 

 made between dorsal and ventral surfaces and 

 between anterior and posterior ends. 



Just as the radiate and biradiate shapes are 

 associated with free-swimming habits, or else with 

 an attached condition, so is the presence of bilateral 

 symmetry similarly connected in its earlier phases 

 with the habit of crawling along the sea-bottom. A 

 most instructive series of gradations is shown by 

 the simpler Turbellarian worms, commencing with 



