244 G. HERBERT FOWLER. 



A " pair " of mesenteries is constituted by two mesenteries whose 

 longitudinal muscle fibres are ranged on their adjacent faces (except 

 in the case of the two " directive pairs," each of which is placed at 

 one end of the longer axis of the mouth oval, and in which the arrange- 

 ment of the muscles is reversed). For the chambers (Radial-taschen, 

 Loges) into which the coelenteron is periaxially divided by the mesen- 

 teries, I am compelled to coin new names. To those chambers which 

 lie between a "pair" of mesenteries the term entoccele is applied 

 (fig. 1, b); to those chambers of which one lies between every two pairs 

 of mesenteries the term exocoele (fig. 1, a). The septa lying in these 

 two classes of chambers are similarly called exosepta and entosepta. 



The classification adopted will be found at the end of the paper, 

 together with the bibliography. 



Recent Researches into the Morphology op the Group. 



In 1873 Lacaze Duthiers (i), studying the development of Astroides 

 calycularis on the coast of Algiers, found that it agreed in every 

 important point with the development of Actinia, his observations 

 on which (2) were corroborated and corrected by the Brothers Hert- 

 wig (3). With regard to the developing skeleton, he recorded two facts 

 of importance — firstly, as appears in his pi. xiv, fig. 27, that it 

 was formed outside the polyp', and secondly, that the theca arose 

 independently of the septa. Owing to various practical difficulties his 

 investigation was incomplete. 



The chief worker in this field has been Georg von Koch, who, in the 

 course of several investigations, has arrived at the conclusion that the 

 theca is a secondary structure, derived from fusion of the peripheral 

 ends of the septa. The evidence adduced in support of this theory 

 appears to me to be at present insufficient for complete proof, though 

 from our slight knowledge of the group it is injudicious to absolutely 

 deny its truth. 



Von Koch first published this theory in 1879,'founding it on the 

 following observations on Caryophyllia (4). There is no living tissue 

 on the greater part of the exterior of the corallum ; but at the apex 

 the peripheral edge of the mouth-disc overlaps the lip of the calyx in 

 such a way that in the highest sections the septa appeared to stand 

 free in the coelenteron, in sections a little lower to have fused 

 peripherally into a theca. The costte are, according to him, and 



