ox MEELIA XOEMAXI. 68^ 



A simple Tnodel such as a wide india-rubbei" tube attaclied 

 at one end and stretched so as to form a " node " — which couhl 

 be tied with string — may serve to represent hyposome and 

 liypersome. If air be forced in at each end, the two sections 

 will become cylinders tending to approximate at the node. 

 The pumped-iu air will represent the growing tissues and cell- 

 masses. Perhaps the rubber tube should be encompassed by 

 a hard cylinder corresponding to the calcareous pit or tube. 



Where growing protoplasm surrounds itself with a nearly 

 closed calcareous cell or wall this ojDposition between the 

 pull of basal inertia and that of centrifugal growth force tends 

 to bring about the formation of a tabula or its equivalent. 



In cases where the soft tissues in a tube are lifted bodily up 

 and secrete a new floor at their base the comjDarison with what 

 takes place in Merlia does not quite hold. The hour-glass- 

 shaped siphonoglyphe of Tubipora and the cystiphragms in 

 certain Monticuliporas appear to be due to special kinds of 

 pulls or strains on the epithelial and other soft tissues. 



The varying style of construction of the calcareous skeleton 

 is due, I believe, to the fact that the calcocytes either settle 

 down and deposit heaps of lumps (the conules of the tubercles) 

 or — in the case of crypt-walls and tabulae — layers of flakes. 

 When crowded in the crypts the actively secreting calcocytes 

 are spread or squeezed out flat and make smooth walls, filling 

 in irregularities between the already formed conules. Some- 

 times from difficulties of terrain, such as the presence of 

 steep ridges or shells, the little builders form thick conical 

 main pillars conulated from base to summit and with scarcely 

 any flanges or tube-walls.^ 



1 The researches of Koch and Bourne seem, to prove that the skeleton 

 of stony corals is formed as a secretion from calicoblasts. Heider and 

 Maria Ogilvie, on the other hand, have concluded that this skeleton is 

 the i^roduct of calcified cells. At present the fonner theory is in the 

 ascendant. I have given evidence, though not so much as I could have 

 wished, to show that the almost coral-like skeleton of Merlia is built 

 up of calcified cells. The amoebocytes of a sponge are so very different 

 from the epithelial cells of a coral that it is doubtful whether the know- 

 ledge of the mode of skeleton formation in Merlia would throw light 

 on the question of stony coral formation. 



VOL. 56, PART 4. NEW SERIES. 48 



