12 MADREPORARIA. 



up of the porous cceaencliymatous walls of the individual polyps, without any trace of epithecal 

 envelopes, or of regular " thecoe."* Hence the application of the term Athecalia. 



The Growth of the Corallum. — We have, then, these two principal factors to account 

 for the special forms of the Turbinarians : (1) the peculiar method of budding, and (2) the 

 gravitation of the fluids thickening the coenenchyma. All or most of the forms wliich the 

 genus assumes can be traced to the varying action of these two. 



A great number of the alterations in the shape of the early cup are due to the folding or 

 frilling of the edge. This, it seems, would follow almost as a necessity from the method of 

 budding. As the circumference of a young cup increases, the distance between any two 

 adjacent radially-developing polyps also increases. Any slight excess of growth in any pair 

 of polyps, before their buds were given off on their under or outer surfaces, would, by in- 

 creasing the space between them, allow a slightly more complete ring of buds to appear ; 

 they would not be entirely confined to the under and outer surface. It is obvious that any 

 appearance of buds ahovc (however slightly) the level of the plane passing through the axes 

 of their parent polyps, must lead to folding of the edge, that is, to an upward folding. A 

 downward folding might occur at any time by the increase in the angle at which the buds 

 leave their parent polyp. By the increase of this angle, also, a corallum which began as a 

 stalked cup might change into an encrusting form. 



With regard to the changes brought about by the downward streaming of the coenen- 

 chyma, we have already mentioned the tliickening of the stalk of the cup which may even be 

 obliterated, and to the gradual fiUing up of the base of the cup inside. In the same way 

 valleys are filled up and fronds thickened. This downward streaming, moreover, leaves un- 

 mistakable signs in the surface markings of the coenenchyma. The very characteristic 

 longitudinal striation on the outer side of the cup, noticed by nearly all those who have 

 described the Turbinarians, indicates the direction of tlie superficial canal system. This stria- 

 tion, or channelling, of the surface is also well marked on the inner side of deep cups ; it 

 indicates the downward streaming which leads to the filling up of the bottom of the cup 

 already described. There are no such channels near the growing edge of the cup, nor in 

 any instance where the fluids are required to build up the coenenchyma in sittt. In these 

 cases tiie cct-nenchyma is always spongy. 



The Turbinarians form no exception to the rule of progressive dying down of the basal 

 regions as the corallum grows. But this is often greatly disguised by the flowings of the 

 coenenchyma above described ; downward streamings of living coenenchyma, often producing 

 polyps, not only submerge living polyps, but overrun the dead basal portions of the corallum. 

 The fixation of successive layers of these downward streamings at difl'erent heights gives many 

 coralla a remarkable appearance, as if there had been alternate periods of growth and decay. 

 But although the Turbinarians, as a whole, conform to the rule of progressive growth, with 



* Whether any essential morphological distinction exists between the regular circular " theca," 

 and the irregular synapticular connections between the septa, I cannot here decide. The distinction 

 is of practical utility. 



