64 ZOOPHYTES. 



from the surface; so that the live portion, could it be separated, 

 would form a thin hollow hemisphere. The depth to which life 

 extends, may, in general, be estimated from the diameter of one of 

 the polyps; for in the Actinia?, as well as the Astra^as and Caryo- 

 phylliaj, the depth (or height) often exceeds but little the diameter, 

 and very seldom, in any species, three diameters. 



Even the branching Madrepores are usually lifeless along the axis 

 of the branches ; and in the Porites, whether forming a branch half 

 an inch in diameter, or a glomerate mass of twenty feet, the polyps 

 do not extend within, beyond two lines. The interior is dead coral, 

 the former animal tissues of which have dried up. 



The branching or columnar coral zoophytes are not only dead 

 along the axis, but they become throughout dead at bottom, after 

 attaining a certain height. The addition of an inch at apex is death 

 to an inch below. Some Goniopores, which grow in columns, two 

 feet or more in height, have a head of live polyps— a capital to the 

 column — of only two or three inches. 



Upon this principle of growing and dying, depends the vast power 

 and geological influence of the coral polyp. But a few lines in 

 height themselves, they would otherwise be limited in their coral- 

 making to as many inches at the most, and what is now styled the 

 coral-garden, would be but a bed of mosses or incrusting lichens. 

 Like the sphagnous moss of a peat-swamp, coral zoophytes continue 

 growing at top, with none the less luxuriance, though supported on 

 several feet of lifeless trunk. Death follows on " sequo pulsat pede" 

 up the stem of a zoophyte " regumque turres." 



The nature of this dying process seems to be simply this : that 

 circulation loses its activity below, as growth proceeds above, and, 

 consequently, the parts dry up in the pores of the corallum. In the 

 Astrseas, this takes place continuously, at the same rate as increase 

 above, and produces a gradual change of the animal. In some Cya- 

 thophyllidis, the same process goes on interruptedly, as explained 

 by Ehrenberg The tissues of the polyp disappear at intervals from 

 the sides, leaving a row of unoccupied cellules; and the animal 

 afterwards goes on to increase from its contracted size, without refill- 

 ing the cellules, which are, therefore, left vacant, though usually 

 closed above at the time of the retraction. Thus the surface of the 

 zoophyte becomes covered with encircling ridges, and the corallum 

 appears to consist of a series of inverted cones inserted one in the 

 other. There is a gradual transition from species, in which these 



