liVDR.K, JKLLVFISHES, SEA ANEMOIS-ES, CORAL POLYPS 51 



colonial animul because it is made up of two kinds of "in- 

 dividuals," or, as they are called, zooids, each kind of zooid 

 having a special func- 

 tion to perform. The 

 zooids are borne on 

 a conunon, slender 

 stem, or axis, and one 

 kind is known as the 

 nutritive zooids while 

 the other kind is 

 known as the repro- 

 ductive zooids. 



The axis may be 

 said to consist of two 

 portions, a horizontal 

 portion which runs 

 over the rocks and 

 weeds in a horizontal 

 direction (Fig. 18, a), 

 and a vertical portion 

 which consists of ver- 

 tical stems arising 

 from the horizontal 

 jjart of the axis. 

 Moreover, the verti- 

 cal stems give off 

 lateral l^ranches in an 

 alternate manner and 

 the nutritive zooiils 

 are borne on the outer ends of these lateral branches. All 

 of the different portions of the axis are about the size of 

 cotton threads, hence this animal is visible to the unaided 



Fig. is. — C:imi';uiularian hydroid : a, horizon- 

 tal Ijrancli ; pr, pcri.sart ; rp. reproductive 

 zooiils 



nf. imtritivf zociid ; c. rcpno-sarc. 



