^^"^ ZOOGENESIS "^""^ 



capable of locomotion, but are sluggish and creep 

 equally well in any direction. Some, like the jelly- 

 fishes (figs. 3, p. 5; 78, p. 143) and the ctenophores 

 (fig. 66, p. Ill), float freely suspended in the water. 

 Others are elongated and more or less bilaterally 

 symmetrical, like the Venus' girdle (Cestus) and the 

 creeping ctenophores. Some are naked, while others 

 construct beautiful and complicated bases, like the 

 corals, or are even completely enclosed in pro- 

 tecting plates (Primnod), or live in tubes of sand or 

 mud. Thus these creatures repeat the bodily forms of 

 the protozoans so far as their structure will allow. 



Among the bilaterally symmetrical animals derived 

 through a gastrula stage many are attached, as nearly 

 all the polyzoans (figs. 67, 68, p. iii), most sea-lilies 

 (fig. 6, p. 5), many crustaceans, like the barnacles 

 (fig. 30, p. 47), most tunicates, and certain other 

 types; some of the attached forms, as certain of the 

 polyzoans, form colonies on the summit of a long 

 stalk. Others, as most sea-urchins and starfishes, are 

 free living and capable of locomotion, but are sluggish 

 and creep equally well in any direction. Some, as 

 certain rotifers, tunicates and holothurians, float 

 freely suspended in the water. Very many are bilater- 

 ally symmetrical and swim, run or fly with great 

 rapidity. Some are naked, while others, as the bar- 

 nacles, brachiopods, mollusks and polyzoans, form 

 regular and complicated limy shells or, as in the case 

 of many worms, some rotifers, some phoronids, and 

 some insect larvas, construct rough agglutinated tubes 

 of sand grains or other forms of body covering. 



^53] " 



