36 ZOOLOGY 



is captured and pushed through the mouth into the interior 

 of the tube, where it is digested. These tentacles are muscular 

 prolongations of the body capable of exceedingly quick move- 

 ment, and containing in their outer cells numerous stinging- 

 cells or cnidoblasts. These stinging-cells are just as char- 

 acteristic of Coelenterata as the collar-cells are of Porifera. 

 Each cell has a little projecting process or sense-hair for 

 perceiving stimuli: whilst embedded in its cytoplasm is a 

 little bag, termed the nematocyst, which contains a clear 

 fluid, and one end of which is tucked inwards into its 

 interior like the turned-in toe of a stocking. This end is 

 very long, and is termed the thread. When the animal 

 is irritated, the cells forming the outer layer of the body 

 contract, and squeeze the stinging cell. As a consequence the 

 nematocyst of the stinging-cell is squirted out like a seed from 

 a ripe orange, and the in turned end or thread of the nematocyst, 

 is also turned inside out. This thread is coated with a 

 poisonous substance, and it penetrates the bodies of the 

 animals on which the Ccelenterate preys, and stuns and kills 

 them. In most cases the thread is incapable of penetrating 

 the human skin, but in the case of some of the larger jelly-fish 

 it can pierce even this, and then the sufferer experiences a 

 burning pain. 



"Coral" is a general name given to the hard skeleton 

 of Coelenterata when this is present : it is formed in many 

 different ways. The beautiful red coral, which is used to 

 form bracelets and necklaces, consists of needles of calcareous 

 matter cemented together; these needles are formed just 

 like the spicules of sponges by cells which have migrated into 

 the jelly from the outer layer of cells. The coral, which 

 forms the masses of which coral reefs mainly consist, appears 

 to be formed as an external exudation from the cells of the 

 external layer. In this way cups or thecse are formed, in 

 which the bodies of the Coelenterate individuals are contained. 

 To the fanciful eyes of the ancients these thecse resembled 

 flowers, and so the legend arose that the branches of coral 

 were a subaqueous flowering plant, which became transformed 

 into stone when it was exposed to the air. 



Next in degree of complication of structure to the Coelen- 

 terata comes the phylum of Platyhelminthes or Flat-worms. 



