DEVELOPMENT OF CORAL POLYPS. 



83 



spermatic bodies rupture the walls of their respective glands 

 situated on the fleshy partitions. As in Actinia, Lacaze- 

 Duthiers thinks the fecundation of the egg occurs before it 

 leaves the ovary, when also the segmentation of the yolk 

 must take place. Unlike the embryo Actinia, the ciliated 

 voung of the coral, after remaining in the digestive cavity 

 for three or four weeks, make their way out into the world 

 through the tentacles. The appearance of the young, when 

 first observed, was like that in Fig. 55, A, being an oval, 

 ciliated gastrula with a small mouth and a digestive cavity. 



The gastrula changes into an actinoid polyp in from 

 thirty to forty days in confinement, after exclusion from the 

 parent, but in nature in a 

 less time, and it probably 

 does not usually leave the 

 mother until ready to fix 

 itself to the bottom. 



Before the embryo be- 

 comes fixed and the tentacles 

 arise, the lime destined to 

 form the partitions begins 

 to be deposited in the enclo- 

 derm. Fig. 55, C, shows the 

 twelve rudimentary septa. 

 These after the young polyp 

 or " actinula" has become 

 stationary, finally enlarge 

 and become joined to the 

 external walls of the coral 

 now in course of formation 

 (Fig. 55, C, c), forming a groundwork or pedestal on which 

 the actinula rests. D represents the young polyp resting 

 on the limestone pedestal. 



Lacaze-Duthiers found that the embryo polyp which had 

 been swimming about in his jars for nearly a month, sud- 

 denly, within the space of three or four hours after a hot 

 sirocco had been blowing for three days, assumed the form 

 of small disks (Fig. 55, B), divided, as in the Actinia, into 

 twelve small folds forming the bases of the partitions within. 



Fig. 55. Development of a coral polyp. 

 Astroidi-s ccdycularis. A. ciliated gastrola: 

 B, young polyp with 12peptii: (7, D. young 

 polyp farther advanced, with 12 tentacles: 

 c, the corallmn and limestone pepta begin- 

 ning to form. After Lacaze-Duthiers. 



