THE SCIENCE OF LIFE. 



awhile by budding, but at a certain period gives birth 

 by subdivision, to free swimming Medusae, or Jelly-fish 

 Each of these, after pursuing for a time its own course 

 of life and development, produces eggs which change 

 into ciliated bodies (Planuld) similar to some of the In- 

 fusoria. After a while each of these becomes stationary, 

 fixes itself to some weed or rock, and becomes a polyp, 

 or Hydroid. 



Those Medusae which swim by the contraction of their 

 umbrella-like disk were formerly called Pulmogrades ; 

 those which swim by vibratile cilia attached to arms, 

 Ciliogrades ; those which float by an expansive bladder, 

 Physogrades ; and those furnished with arms, or cirri, 

 Cirrigrades. Another classification divided them into 

 " naked-eyed " and " hidden-eyed " Medusae. Since 

 more thorough research has shown their relation to the 

 Hydroids, the Medusae have been considered in reference 

 to the families of Hydroids from which they springe 



The Tubularian family (tubulus, a little tube) consist 

 of Hydroids, sometimes simple, but generally compound, 

 united by a common trunk or coenosarc, which has an 

 external horny coat, or polypary. Sometimes the tube 

 is jointed with the tentacles placed in a whorl round each 

 joint, (Tubularida divisa^} sometimes it is undivided, 

 (71 indivisa^} Sometimes the polypary is much branched, 

 (as in Eudendrium?) but in the majority it is not branched. 

 A few species have no hard polypary, (as Corymorpha 

 nutans^) but simply a white fleshy stem. The polyps of 

 this family have no protecting cups. The Medusae bud 

 from the stem. 



The Sertularian family (Sertula, a little wreath) is gen- 



