42 ANIMAL FORMS 



colonies are produced (Fig. 26). At the same time the 

 outer layer of the body is continually forming a skeleton 

 which encloses the colony as a sheath, except at the ter- 

 mination of each branch, where the mouth and tentacles 

 are located. In certain species for example, the sea pens 

 (Pennatula) and sea fans (Gorgonid) a skeleton may be 



FIG. 26. Small portions of 



with some of the polyps expanded. 



formed of myriads of lime spicules, somewhat like those 

 of the sponge, which are bound together by the fleshy 

 substance of the body; but the skeleton of most of the 

 common forms in the ocean, and the coral found in 

 general collections, is stony. According to their method 

 of branching, such specimens have received various popu- 

 lar names, such as brain, stag-horn, organ-pipe, and fun- 

 gous corals. 



