CBRIANTHARIA. 



37 



which were remarkably well preserved and in the same stage of development (with 17 mesenteries), 

 to establish the fact that the arrangement which occurs in the adult Ceriantharia, is found also in 

 A. albida. The muscle arrangement on the mesenteries was specially clear in one specimen, which 

 was strongly but uniformly 1 ) contracted throughout. All the mesenteries including the directive 

 had the transverse muscles extended upon the side towards the directive chamber, whilst the longi- 

 tudinal muscles were developed on the side remote from them (fig. 8, PL 5). My diagram (1893 a) of 

 the muscle arrangement on the mesenteries will thus be of value even in the matter of the directive 

 mesenteries. 



Systematic Remarks. As I have already pointed out, this larval form is possibly that of Arach- 

 nantlius sarsi. The dissimilarity of structure in protomesenteries 2 is sufficiently great however, to 

 compel us to leave the question undecided. In any case A. albida is a larval form falling under the 

 same genus as Arachnanthus, which cannot be said of Arachnactis bournei and brachiolata (compare 

 p. 41). For further treatment of this larval form I refer to "Nordisches Plankton". 



Section III. 

 Classification of the Ceriantharia. 



As the earlier descriptions of Ceriantharia are very imperfect, and indeed some of the more 

 recent also, it is particularly difficult to obtain a fair conception of a good many Cerianthid species. 

 It is extremely probable of course that many of these are genuine species, for the Ceriantharia do not 

 seem to be so deficient in species as was formerly believed, and as Pax (1910) -- though without 

 close study of the facts still thinks probable. That there must be several species shewing con- 

 siderable divergencies in the adult state, may be concluded moreover from van Beneden's, in many 

 respects, meritorious work on Cerianthid larval forms in the Plankton Expedition (1898). Van Be- 

 neden shewed that two distinct types of larval forms might be distinguished: 



(1) Acontifcres having acontia. 



(2) Botrucnidiferes, provided with special stinging organs called Botrucnidae. 

 As genera of Acontiferes van Be ne den accounted: 



Arachnactis, Ovactis, Dactylactis, Solasteractis Apiactis, Peponactis, 

 As genera of Botrucniferes: 



Cerianthula, Hensenanthula, Calpanthula. 



In Solasteractis, Apiactis and Peponactis van Beneden found no acontia. Still he did not 

 make a distinct group of them, doubtless for the reason that he believed, that these larval forms would 

 also develop acontia at a later stage. For in his diagnosis of these three genera we find the attri- 

 bution "Aconties absentes on tardives". 



") The mesenteries were also strongly contracted similarly to the column. Such specimens are plainly the best for 

 the study of the mesenterial musculature, whilst the specimens which shew strong contraction of the column without con- 

 traction of the mesenteries are the worst for purposes of research. For in the latter the mesenteries become so folded 

 through the contraction of the column that it is particularly difficult to get an idea of the arrangement of the musculature. 



