TUEIPOKA MTJSICA COEALS. 



223 



(fig. 7) ; these, when the animal is alive, help to support its 

 gelatinous frame ; the Tubiporse also "belong to this class. 



FIG. 7. SPICULES OF GOEGONIA, MAGNIFIED. 



Pig. 8 represents the Organ-coral (Tubipora musica) j 

 it is of a bright red colour, and, when the polypes are 

 living, is a very beautiful object. 



The third order of Zoophytes is the Helianthoida, which 

 include the " Actinia." These Zoophytes are entirely gela- 

 tinous, and have beautiful variations of colour. They throw 

 out tentacles like the petals of a flower, and form very beau- 

 tiful objects ; they may be seen in the aquatic vivaria of the 

 Zoological Gardens. To this order of Zoophytes belong the 

 "corals" or Zoanthoid polypes, which produce the coral 

 reefs, extending in some cases hundreds of miles. Dr. Baird 

 gives the following account of them : 



" By far the greater part of the Zoanthoid polypes, as they 

 grow, deposit in the cellular substance of the flesh of their 

 back an immense quantity of calcareous matter which enlarges 

 as the animal increases in size, and, in fact, fills up those por- 

 tions of the substance of the animal, which by the growth 

 of new parts are no longer wanted for its nourishment, and 

 in this manner they form a hard and strong case, amongst 

 the folds of which they contract themselves so as to be pro- 

 tected from external injury, and by the same means they 



