78 SIDERASTREA RADIANS. 



actinians,* though the evidence for the Cerian these and Zoantheae is not yet 

 conclusive. 



The length of time the young polyps of Siderastrea remained with 

 only twelve mesenteries eight inacrocuernes and four microcnemes would 

 indicate that the stage is one of phylogenetic, as well as ontogenetic, 

 significance. No marked resting stage in the mesenterial development 

 occurred until this was reached, while none of the polyps showed further 

 progress until the expiration of three or four weeks, and some of the smaller 

 polyps remained thus for three months. Further, the fact that four pairs 

 united with the stomodseuni, while two pairs remained separate for the whole 

 seventeen weeks, the six pairs of second-cycle mesenteries arising in the 

 meantime, would suggest a different significance for the complete and incom- 

 plete groups (8 + 4). 



The relationship, 8 + 4, is characteristic of the larvae and young polyps 

 of Madreporaria and Actiniaria generally, and persists in some species 

 throughout the life of the polyp. Only the first four pairs of mesenteries 

 are ever united with the stornodseum in the Edwardsidse, Gonactinia, and 

 some other Actiniaria, and in the mature polyps of all West Indian species 

 of Madrepora and Porites the same eight protocnemes alone are complete, 

 and the other four remain incomplete. 



The long retention of freedom of the fifth and sixth pairs of protocnemes 

 suggests to my mind an ancestry in which the mesenteries as a whole, includ- 

 ing the metacnemes, were alternately long and short, excluding, of course, 

 the axial directives. Among modern examples this is retained in the mesen- 

 terial system of the zoanthids, Porites, and Madrepora, and was perhaps 

 characteristic of the Rugosa. The arrangement of the musculature on the 

 mesenteries in the Cerianthese, always on the face towards one aspect of the 

 polyp, can be also understood if one considers that the incomplete mesenteries 

 present in such forms as the zoanthids are never developed ; the cerianthids 

 retain the simplest protocnemic stage of an}' of the forms here considered, 

 having only four bilateral pairs. 



In the Cerianthese and Zoanthese the mesenteries beyond the protocnemes 

 arise at only one or two restricted regions of the polyp (bands of proliferation), 

 not all round the circumference, as in most modern anemones and coral 

 polyps ; and four such zones or bands of proliferation would appear to have 

 been characteristic of the mesenterial growth in Palaeozoic coral polyps. The 



* AppellSf (1900) has discussed fully the value to be assigned the accounts of Lacaze-Duthiers, Haddon, 

 and others as to various other sequences of the primary mesenteries in actinians. Throughout their work 

 Delage and He>ouard (1902) transpose the order of the fifth and sixth pairs as given above. 



