ALCYONARIA 185 



as three or four metres , and contains a long horny axis which is possibly 

 endodermal. The secondary polyps are budded off from endodermal 

 tubes which ramify in the much thickened mesogloea of the body wall 

 of the primary polyp, and belong to two types of individuals, the 

 normal autozooids which feed the colony and the stphonozooids, with 

 reduced mesenteries and enlarged siphonoglyph, whose only function 

 is to maintain the circulation of water in the canals of the colony. The 

 autozooids in Pennatula are arranged in rows side by side to form 



Fig. 138. Gorgonians (two species on the left) and hydrocorallines (on the 

 right) growing on a coral reef in Florida. From an underwater photograph 

 by Professor W. H. Longley. 



equal and regular lateral branches on each side of the axis giving the 

 colony its feather-like form, and the siphonozooids are mainly found 

 on the back of the axis. A colony has a limited but remarkable power 

 of movement and can burrow into sand or mud by its basal stalk. 



In two genera, Tubipora (the organ-pipe coral) and Heliopora (the 

 blue coral), which are widely distributed on coral reefs, a continuous 

 calcareous skeleton is developed resembling that of reef corals. The 

 polyps of Tubipora are elongated and parallel and connected by stony 

 platforms which are traversed by the endodermal tubes. But while 



