IV 



COELENTEEATA 



87 



(1901), have added nothing of any importance. Wilson worked on 

 the species Renilla reniformis and Leptogorgia virgulata (1883). 

 The eggs are expelled from the parent colonies and fertilized in the 

 sea. The endoderm appears to be formed by proliferation, and all 

 the eight mesenteries appear at once. The arrangement of the 

 muscular thickenings on these mesenteries differs only from that in 

 the Edwardsia stage of Hexactiniae, in the fact that the thickenings 

 of the ventral directives face each other instead of being turned away 

 from each other. Wilson's observations on the origin of the filaments 

 have already been dealt with. 



The Alcyonaria differ less in this respect from the Edwardsiae 

 than these differ from the Cereanthidste, and the real ground of their 

 separation from the Zoantharia 

 lies in. the method of forming 

 the skeleton, as will now be 

 made clear. 



The popular name Sea- 

 Anemone is usually given to 

 those Zoantharia which do not 

 develop a calcareous skeleton, 

 whilst those which do form 

 skeletons are termed Coral - 

 forming Polyps — or briefly. 

 Corals. The development of 

 the skeleton in these was first 

 worked out by Lacaze-Duthiers 

 (1864), but the subject was 

 again taken up and thoroughly 

 examined by von Koch (1897), 

 whose latest investigations 

 deal with the Mediterranean 

 species CaryophyUia cyathus. 



The first part of the skeleton to appear is the basal plate which 

 is secreted by the pedal disc of the polyp. This basal plate appears 

 as six separate areas of calcareous deposit, one area being situated 

 beneath the space intervening between each pair of mesenteries. 

 They eventually coalesce to form, first a six-rayed star and then a 

 circular disc. In the centre of each of the original areas the process 

 of secretion of calcareous matter continues more actively than else- 

 where and the consequence of this is the formation of six radiating 

 septa 'of calcareous matter, each septum being covered by an inwardly 

 proiecting fold of the pedal disc. The edge of the basal plate 

 becomes raised into a rim, owing to the upward extension of the 

 skeleton-forming area on the side of the polyp, and m this way the 

 beginning is made of a theca or cup in which the polyp sits. 



Both septa and thecal wall grow in height : soon a set ot 

 secondary septa alternating with the primary ones mak 

 appearance, whilst in the centre of the basal plate there appear two or 



Fia. 67. — Young living Caryopliyllia cyathus 

 seen from above. The calcareous skeleton 

 shows through the transparent tissue. (A fter 

 von Koch.) 



Letters as in preceding figures. In addition, s, one 

 of the primary septa ; th, wall ot theca. 



SIX 



their 



