38 . SIDERASTREA RADIANS. 



followed by the different species of corals; some will be protandrous while 

 others will be protogynous. 



It is clear from the evidence adduced that a dioecious character can not 

 be assumed from the presence of only ova or sperm aria until the entire 

 developmental history of the species has been followed ; a polyp having at 

 one time only one kind of sexual cells may later show both kinds. 



. 



CORALLUM. 



Freshly macerated coralla are generally white or yellowish in color, but 

 frequently they are somewhat green, either as a whole or in patches. On 

 breaking a green corallum across the coloration is found to extend some 

 little distance below the surface, and, after decalcification, there remains 

 behind a green mass of fluffy texture, which, on examination under the 

 microscope, is found to consist of filamentous algae. Bven where the skeleton 

 presents no superficial green coloration, a filamentous mass remains behind 

 on decalcification, and chlorophyll may be present in small quantities, or the 

 filaments may be dead and altogether devoid of protoplasmic contents. Under 

 the microscope fragments of the skeleton frequently show fibers crossing the 

 interseptal spaces or meandering over the surface, and sections of the corallum 

 are sometimes perforated by similar filaments (plate u, fig. 66). Thus 

 the algae actually bore their way into the skeleton as well as spread over the 

 surface. They are particularly abundant in decaying masses of coral. 



The coral-boring algae liberated by decalcification are represented by 

 both a non-septate and a septate form, the filaments of both varying greatly 

 in size. Associated with the algal filaments are clusters of a LeptothrixAV&s. 

 bacterium ; but whether its threads actually perforate the skeletal matter, or 

 merely grow over its surface, living upon the traces of organic matter, seems 

 uncertain. In a former paper I have pointed out the almost universal 

 presence of boring algae in corals, and their importance in disintegration 

 (Bull. Amer. Mus. Nat. Hist, xvi, 1902). 



In addition to the parasitic algae, organisms such as cirripedes, molluscs, 

 or tubiculous worms usually occur, either inclosed within the skeleton or 

 adherent to its surface. These associated organisms are particularly numer- 

 ous on specimens obtained from muddy shores as compared with those from 

 the clearer waters of the reefs, and often interfere with the regularity of form 

 of the corallites of the colony. The cavities resulting from the activity of 

 the boring molluscs weaken the skeleton greatly, so that the colonies can be 

 readily broken into fragments. 



Numbers of the parasitic cirripede Pyrgoma sometimes occur, exhibiting 



