284 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



gastric cavities of the separate polyps communicate 

 with these canals. The coralla, though porous, are 

 very hard and stony, and the canals are separated into 

 storeys by tabulae, and the upper chambers are alone 

 living. The Millepores are common on coral reefs, the 



Fig. 118 B, Coral. Caryophyllia cyathus. 



Stylasters live in water from ten to seven hundred 

 fathoms in depth. 



In all the other Hydrozoa the supporting tissue is 

 simply a supporting lamella mesodermal in position, 

 as in the common Hydra, or there is an outer chitinous 

 perisarc, as in many Hydroid polyps, which persists 

 after the death of the animal as the so-called coralline ; 

 or, as in the Medusae, the tissue lying between the 

 ectoderm and endoderm becomes gelatinous (Fig. 119), 

 or cartilaginous. 



Spicular skeletons are not found among Verities, 

 where, when a protecting tube is developed, it is often 



