ij6 Practical Zoology. 



form, then, the secretion is wholly in the living matter between 

 the polyps, the barklike layer being composed of the dried flesh 

 in which the spicules lie embedded. 



Strip off a piece of the barklike 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 run- 

 ning lengthwise near the inner surface of this layer. Find 

 the openings of the tubes where they were broken ; these tubes 

 connect the polyps of the colony. 



2. The central axis of hornlike substance. Test its flexibility 

 and strength. Observe the grooves on its surface, and the 

 relation between them and the tubes above noted. This horny 

 axis is excreted by the walls of these tubes, and is not pene- 

 trated by living matter like the outer layer. In the precious red 

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

 ous instead of horny, and the outer barklike 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 ten- 

 tacles ; also eight mesenteries, which are never in pairs. An 

 alcoholic specimen, with the polyps expanded, should, if pos- 

 sible, be examined. 



Topics for Reports. Coral Islands and their Formation. 



