SEA-FEATHER, OB SEA-FAN. 175 



the spicules is, to thoroughly clean them by boiling 

 some of the outer laj^er 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. 



