110 Professor Yakovlev — On Rugose Corals. 



And in this respect, as well as in its general form, the coral already 

 shows an external bilateral symmetry. Scars of attachment and 

 root-like processes, both on the convex side, and a general external 

 bilateral symmetry, characterize all the known Rugosa from the 

 Silurian rocks, the earliest in which they have yet been found, to 

 the Permian in which the last Rugosa occur. Further, when, owing 

 to the character of the sea-bottom, there is no possibility of attach- 

 ment and the coral consequently is free, the polyparium, whether 

 of a Rugose or Hexa-coral, takes on a flat, discoid shape (Palaocyclus, 



'* Microcyclm, etc.). But, in spite of its not leaning over, the Rugose 

 Coral retains its bilateral symmetry, shown externally chiefly by the 

 apex which lies excentrically, while that of the Hexacoral is central. 

 That is to say, in spite of the removal of the conditions producing 

 bilateral symmetry, the discoid Rugose Coral exhibits it. Is not this 

 an example of the inheritance of acquired characters? 



Another form showing the tendency to retain bilateral symmetry 

 after the conditions which caused it have been l'emoved is that 



\ taken by unattached genera such as Calceola, Platyphyllum, and other 

 operculate corals. These are curved, flat on the lower side, and 



Text-fig. 1. — Diagram representing two polyparia of solitary Kugosa growing 

 side by side. The arrows outside the polyparia indicate the direction of 

 the prevailing current, and those within the calices that of the water along 

 the channels formed by the fossulas. H, G, S, the positions of the 

 Main, Counter, and Alar fossulse respectively. Nat. size. 



provided with an operculum whose function is to prevent the 

 penetration of mud into the coral's calice. Operculate corals are 

 probably polyphyletic in origin, and, as would be expected, occur 

 only among the Rugosa. 



The last external character dealt with is the frequently occurring 

 phenomenon of "Rejuvenescence". The term is inappropriate, since 

 young individuals also are subject to it. Rejuvenescence consists of 

 a periodically repeated retraction of the calice as if by shrinking or 

 by the appearance of a daughter coral produced by intracalicinal 

 budding. Rejuvenescence, especially in the broadly conical forms, 

 is easily explicable, in fact only to be expected, when it is con- 

 sidered how inconvenient and disadvantageous from a mechanical 

 point of view a considerable upward expansion of the polyparium 

 would be to the Rugosa with their typically conical polyparium. 



Turning from the external to the internal characters of the Rugose 

 skeleton, we shall find that they are determined by the curved 

 condition of the conical polyparium. We shall consider, first of all, 

 the characteristic arrangement of the secondary septa and their 



