CKRIANTHARIA. 



57 



their development -- we may even assume a priori that the order of successive appearance of the 

 first 8 mesenteries in the different Anthozoa may many times have undergone modification. 



Besides, it may be questioned, whether any difference in the time of development of the second 

 and third mesentery couple, both of which form at the corners of the mouth in the 6-mesentery stage 

 of Ceriantharia, really takes place at all: from all we know it must be very slight, if it exists at all. 

 In any case we must very cautious in making use of the order of succession of the first 8 mesenteries 

 for the orientation of the body in Anthozoa, because the smallest difference of time, whether it means 

 the anticipation or retardation of the appearance of a couple, may completely upset the whole order 

 of development of the mesenteries or else bring the originally different orders of mesenterial develop- 

 ment shewn by two groups, into harmony with each other. In such circumstances we run the risk 

 of making serious mistakes if we make use of the order of development of the 8 primary mesenteries 

 as evidence for the orientation of the body in Anthozoa. 



In the order of appearance of the subsequently formed mesenteries in Ceriantharia, Zoantharia, 

 Actiniaria and Madreporaria a certain agreement is found, which certainly is not very obvious at first 

 sight in all groups, but still does occur according to the results arrived at by several investigators, 

 even in biradial Anthozoa; this consists in the tendency of the mesenteries to develop from the 

 one couple of directive mesenteries towards the other. The Ceriantharia from the 6-mesentery stage 

 develop their new mesenteries in a zone lying just opposite the siphonoglyph, and the formation of the 

 new mesenteries takes place from the siphonoglyph towards the opposite side. The Zoantharia from 

 the i2-mesentery stage perhaps even from the 8-mesentery stage develop the mesenteries from the dorsal 

 side towards the region of the siphonoglyph, that is from the i2-mesentery stage in two zones, one 

 on each side of the large ventral couple of directive mesenteries. In the development of mesenteries 

 8 to 12 in Actiniaria and Madreporaria, as also in some cases in the appearance of the mesenteries of 

 the second and third cycles, we may trace a development of the mesenteries from the dorsal side 

 towards the ventral, in the fact that the mesenteries of the different cycles form earlier in the dorsally 

 situated exocoels than in the ventral. [Carlgren (1893, 1897) Faurot (1903), Duerden (1902)]. 

 Whether this phenomenon in the biradial forms is the rule, it is no doubt too early to make sure, 

 since hitherto comparatively few forms have been examined, but even if it were not the case the fact 

 would not signify much, because we must of course assume that in many cases the rearrangement 

 of the mesenteries from a bilateral to a biradial symmetry has obscured the course of development 



Thus we can plainly trace in the development of the mesenteries in the above mentioned 

 Anthozoa a formation of the mesenteries from the one side towards the other. Now if we fix the 

 orientation of Ceriantharia as the mesenterial musculature requires, we obtain also a development of 

 the mesenteries from the dorsal towards the ventral side, and consequently a general conformity of 

 which we may always expect to find indications in all Anthozoa that have numerous mesenteries. 



We have thus two factors, the mesenterial musculature and the order of development of the 

 mesenteries after the (6) 8-mesentery stage, which tell in favour of the view that the directive couple 

 of mesenteries in Ceriantharia are dorsal (anterior) and that the single siphonoglyph is here a sulculus. 

 Of these the first factor is probably sufficient to determine the orientation of the body in Anthozoa 



The Danish Ingolf-Expedition V. J. 8 



