SEA-FEATHER, OR SEA-FAN. 175 



the spicules is, to thoroughly clean them by boiling 

 some of the outer layer in caustic potash. In this 

 layer are holes from which the polyps protruded. In 

 this form, then, the secretion is wholly in the living 

 matter between the polyps, the bark-like layer being 

 composed of the dried flesh in which the spicules lie 

 imbedded. 



Strip off a piece of the bark-like layer and note the 

 grooves on its inner surface. By examining the end 

 of this piece it may be seen that these grooves are 

 caused by a series of tubes running lengthwise near 

 the inner surface of this layer. Find the openings of 

 the tubes where they were broken ; these tubes con- 

 nect the polyps of the colony. 



2. The central axis of horn-like substance. Test its flexi- 

 bility and strength. Observe the grooves on its sur- 

 face, and the relation between them and the tubes 

 above noted. This horny axis is excreted by the 

 walls of these tubes, and is not penetrated by liv- 

 ing matter like the outer layer. In the precious red 

 coral the central axis is formed in the same way, but 

 is calcareous instead of horny, and the outer bark-like 

 layer has been removed. 



3. Note the mode of branching in a sea-fan, comparing 

 the margin with the central portion to see how the 

 meshes are formed. Remove some of the outer layer, 

 and compare with the sea-feather. In this group 

 (including sea-feathers, sea-fans, the precious red coral, 

 etc.) each polyp has eight fringed tentacles; also 

 eight mesenteries, which are never in pairs. An alco- 

 holic specimen, with the polyps expanded, should, if 

 possible, be examined. 



