ALGYONIUM. 99 



above-mentioned. A careful examination of the body- 

 wall of the polyp heads with a microscope (PI. II., fig. 15) 

 reveals the fact that it consists of three layers, an outer 

 layer of cells — the ectoderm, a middle homogeneous layer 

 — the mesogloea and an inner layer of cells — the endo- 

 derm. If these three layers be traced down to the surface 

 of the colony it will be found that the ectoderm is con- 

 tinued over the general surface of the colony as a covering 

 or protective sheath, that the mesogloea is continuous 

 with the gelatinous mass or mesogloea common to the 

 individuals composing the colony, and that the endoderm 

 forms an inner lining to the tubes almost to the base of 

 the colony. 



The colony then is formed of a number of individuals, 

 each of which consists of two parts — a greater part below 

 bound to its neighbours in a common mesogloea, and a 

 smaller part above which is free. 



This latter part can be introverted into the former 

 for protection — in much the same way as the head of the 

 tortoise can be withdrawn into the shelter of the carapace 

 —and it may be distinguished by the name " anthocodia " 

 suggested by Mr. Bourne. When the "anthocodia" is 

 retracted the aperture of the tube or false mouth can be 

 constricted and closed so as to give complete protection 

 to the polyp, as seen by reference to Plate I., fig. 3, m 

 which a series of stages of the retraction of the anthocodise 

 is illustrated. This power which the colony possesses of 

 completely closing the "false mouths" of the polyps is 

 of some physiological interest, as it enables each polyp of 

 the colony to retain in its cavity a sufficient supply of 

 sea-water to maintain its vitality for a few hours 

 when the tide is exceptionally low and the colony 

 is exposed to the air. Without this power the delicate 

 cells which cover the tentacles and body - wall of 



