ORDERACTINOIDEA. 63 



zoophyte, arising from the fact that the coral secretions take place 

 only from the lower parts of the polyps. 



It is hence apparent, that by this single difference in growth, the 

 same mode of budding may produce either massive forms — globular 

 or ramose — or a branched zoophyte, in which each branchlet is the 

 growth of a separate polyp. The former mode of growth produces 

 what may be styled aggregate zoophytes, as is exemplified in figure 23, 

 and in the various genera just referred to. The latter gives rise to 

 segregate zoophytes, the polyps being separate from one another, 

 excepting a basal connexion. The coralla, in the latter case, may be 

 described as calicularly branched. This subject will be farther illus- 

 trated when treating of tlie modes of budding. 



b. 61. Germ-polyps differ essentially in their mode of increase, — a 

 process intimately connected with that of budding. The adult ani- 

 mal, commencing a zoophyte, sometimes seems to raise itself on the 

 coral it secretes, and, although but a fraction of an inch in height 

 itself, gives rise to stems many times its own length. This is the case 

 with the species represented in figure 24, in which only the tips of 

 the branches, for a line or less, are alive. The part below dies as 

 growth proceeds above, and so growth and death go on at equal pace till 

 the small polyp is finally supported upon a long pedicel of dead coral. 

 This is styled an acrogenous^ mode of increase. In the example 

 referred to, and many others among coral zoophytes, there is no con- 

 nexion whatever between the several polyps of the group, except for 

 a short time after a bud first starts, the process of growth causing 

 after awhile a complete separation between each bud and its parent. 



Other species are incapable of this indefinite upward growth ; 

 and these consequently scarcely exceed their adult size in height, 

 above the point from which they start into existence. Yet many 

 of these give origin to large zoophytes. Instead of increasing upward, 

 they extend themselves laterally, or rviden by growth and budding. 

 This result may be connected both with lateral and terminal budding, 

 as shown in figures 27 and 33. 



62. The singular process of dying below, while upward increase 

 goes on, is illustrated by nearly the whole class of coral zoophytes, 

 and may receive here some fartlier illustration. 



An Astraea dome, twelve feet in diameter, although solid coral 

 through its interior, is alive for only half or three-fourths of an inch 



* From axpov, top, nnd yewaw, to increase. 



