7. Anthozoa (incl. Hydrocorallia). C. Hydrocorallia, 41 



tubular formation due to inherent causes. S. is descended from incrusting 

 forms and is nearly related to such Cornularids as Sympodium and Erythro- 

 podium. Probably on account of unfavourable conditions the colonies raised 

 themselves and developed on the inner side of the coenenchyme a layer of 

 closely situated special spicules which served as a support. This corresponds 

 to the axis of Sol., while the undifferentiated side of the coenenchyme corre- 

 sponds to the cortex. In this way the author explains the occurrence of polyps 

 only on the outer or cortical face. Mechanical conditions induced the inrolling 

 of such flat expanded structures. 



Kjaer gives a revised diagnosis of the Heliolitidse which he divides into 

 6 subfamilies - - Proplasmoporinse containing the most primitive genus Plasmo- 

 porella (2 sp.); Proheliolitinse containing the primitive genus Proheliolites (1 sp.); 

 Protarseinse (Protrochiscolithus n. g., Trochiscolithus 1; Protarcea 3), which pro- 

 bably arose from forms like Plasmoporella; Palseoporitinse (PalcBOporites), which 

 probably arose from forms like Protrochiscolithus, the perforate structure here 

 reached its most complex form; Plasmoporinse (Propora 14, Plasmopora 12, 

 Diploepora 2, Pycnolithus 1), which are probably descended from Plasmo- 

 porinse and connect these to Heliolitinse (Heliolites 7, Cosmiolithus 2, Acantho- 

 lithus 1) the most highly specialised forms, which arose polyphyletically from 

 Plasmoporinse. The central slightly thickened skeleton in the branching form 

 of Trochiscolithus shows the primitive growth-form of this genus, while the 

 structure of the peripheral parts - - the bacular differentiation of the skeleton - 

 has arisen secondarily during the growth of the colony. In lamellar and 

 sphseroidal colonies the bacular differentiation occurred from the beginning of 

 the formation of the colony and the primitive skeletal structure has vanished. 

 As the bacular differentiation of T. agrees with the usual skeletal structure 

 of Protaresa (= Coceoseris) and their upper surfaces also agree, the skeleton 

 of these genera must have arisen by similar bacular differentiation of a skele- 

 ton similar to the central skeleton of T., and these genera are probably derived 

 from still older corals with a skeleton of this latter type. The bacular diffe- 

 rentiation in Plasmoporinse and Coccoseridse has arisen independently; it is a 

 result of convergence and shows that this peculiar differentiation probably arose 

 in consequence of external conditions in quite different groups of the Helio- 

 litidae and that it may conceal and change the original structure of the skeleton. 



C. Hydrocorallia. 



See Marenzeller( 2 ) and above p 35 Agassizf 1 ), p37 Agassiz( 2 ), p 38 Finckh 

 dan p 39 Hinde. 



