l8 SIDERASTREA RADIANS. 



full vigorous growth. In addition to this, the death of colonies as a whole, 

 without any obvious environmental cause, is not infrequent. Stanley Gardi- 

 ner has recently drawn attention to this subject. During his investiga- 

 tions on the coral reefs in the Maldives and Laccadives the impression 

 was gathered " that practically all the colonies of a species in any one area 

 died, or that there were only the isolated deaths of individual large colonies," 

 apparently without environmental cause. Gardiner is impelled to believe 

 that " The ripening of the generative organs of a large number of polyp 

 colonies of the same species in a single locality or habitat, followed b}- the 

 subsequent death of all these colonies, is a regular phenomenon." 



In the course of my experience in the coral regions of the West Indies 

 I have met with no instance of the regional death of all the colonies of any 

 species, but frequently individual stocks have been encountered of which all 

 the polyps seemed to be in a state of maceration. This was particularly the 

 case with Poriies astrcsoides. Isolated colonies were obtained seemingly alive 

 and normal in color, but upon examination with a lens no distinct polyps or 

 tentacles were recognizable. The whole of the soft tissues seemed to be a 

 gelatinous mass in process of decay, the coloration being due to the persist- 

 ence of the yellow pigment cells characteristic of this species. Such colonies 

 were under exactly similar conditions to others living around them, and 

 without further investigation any suggestion as to the cause of death of 

 the polyps can be only the merest conjecture. 



My own experience leads me to suppose that coral polyps do not die 

 after the ripening of the generative products, but that, from the same indi- 

 vidual, one series of larvae may follow another, for in numerous instances 

 {Fiwia, Poriies) in which polyps charged with larvae, all at the same stage 

 of development, have been examined, there were still many nearly ripe ova 

 within the mesenteries, as if preparing to give rise to another batch of larvse. 



Sexual reproduction in Siderastrea takes place by the formation of 

 planulae from fertilized ova. The planulse undergo partial development, as 

 far as the appearance of the first four pairs of mesenteries, while within the 

 body-cavity of the parent, and are then expelled. After swimming freely for 

 a shorter or longer time they settle and give rise to young polyps. Larvae 

 were extruded toward the end of June and during July. 



EXTERNAL CHARACTERS ON DECALCIFICATION. 



Certain of the external features of the polyps can be studied only after 

 the latter have been freed from the skeletal matrix. The corallum of Sider- 

 astrea is very dense, and the process of decalcification requires several days. 



