ADULT COLONY. 39 



all stages in the growth of its protective shell. The older specimens are 

 completely inclosed and fused within the corallum, only the mouth of the 

 shell being exposed at the surface of the colony. Its outer surface is covered 

 by an irregular growth of the coral skeleton, and, being raised a little above 

 the general surface of the corallum, gives a marked irregularity to the colony 

 as a whole. The external ridges on the shell of the parasite have a curious 

 resemblance to the septa of the coral, and in the later growths it is somewhat 

 difficult to distinguish one from the other. 



On certain macerated colonies many early stages were obtained showing 

 the manner of attachment of the cirripede within the living calice. At first 

 the presence of the small shell of the crustacean has produced no modifica- 

 tion of the coral ; it is simply adherent to a few of the septa. But later the 

 lower ridges on the surface of the cirripede, which closely simulate septa, 

 became fused and partly overgrown with the septa, thus causing the oblitera- 

 tion of the middle of the calice. In every case the intruder has fixed itself 

 wthin a calicinal cavity, never on the ridge connecting two calices. Appar- 

 ently the cirripede larva enters the polypal cavity and then bores through 

 the living tissues to the corallum, and in the process of growth the skeletons 

 of the two become fused. Afterwards the two organisms continue growing 

 together, the coral skeleton almost wholly inclosing the crustacean ; the latter 

 has no boring action. In the later stages the presence of the Pyrgoma 

 results in the production around it of imperfectly developed polyps. 



On breaking through a colony it is frequently found that the corallum 

 is made up of growths of different periods, arranged upon one another in 

 irregular laj'ers, instead of a continuous increase in thickness from the middle 

 to the circumference. An old corallar surface has become dead and partly 

 corroded, and then a later skeletal deposit has been formed upon this, and so 

 on for several successive stages, the vertical axis of the corallites of the two 

 or more periods of growth usually corresponding. The secondary origin of 

 polyps in old calices has been already described (p. 17). The growth of the 

 corallum seems to be more continuous in blocks of the larger S. siderea than 

 in S. radians. 



The actual degree of calcification of the corallum, if one may thus express 

 it, varies much in different colonies, and also in different regions of the same 

 colony. In some examples the septa are comparatively thick, leaving but 

 narrow interseptal spaces, and the calices are shallower than usual, the 

 columella being solid and prominent. In others the septa are thinner, the 

 interspaces correspondingly wider, the calices deep, and the columella either 

 absent as a definite projection or represented by one, two, or three prominent 



