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of the two distal cycles correspond to the brachycnemes. The marginal tentacles correspondino- 

 to the protocnemes and the labials do not, however, present this relation, and its value as 

 a determining factor must therefore remain uncertain. 



The Mesenterial Filaments. The structure of the mesenterial filaments of the 

 Cerianthese has been considered by von Heider (1879), the Hertwigs (1879) and van Beneden 

 (1898). Von Heider noted the occurrence in C. membranaceus of a thickened border to the 

 mesenteries, and observed, too, that the thickening extended to a much lower level upon the 

 fertile mesenteries than upon the sterile ones, the edge of the mesentery below this thickening 

 being occupied by a much smaller swelling. Furthermore he observed that toward its extremity 

 the upper larger thickening on the sterile mesenteries became much contorted and gave off 

 numerous branch filaments, which he identified with acontia. Similar branch filaments, though 

 less numerous, were observed also on the fertile mesenteries and they were found to occur 

 scattered along almost the entire length of the telocnemes. 



The Hertwigs added to von Heider's description a much more accurate account of 

 the finer structure of the filaments. They figure on PI. VIII, fig. 12 of their paper the upper 

 portion of a mesentery, showing the upper portion of the filament but slightly wavy and bearing 

 numerous slender, branched processes, this portion being succeeded by a stretch in which the 

 filament is greatly coiled. Below this the filament is described as running in a wavy course to 

 the end of the mesentery. In its upper part the filament is shown to consist of three clearly 

 differentiated portions, a median " Nesseldrüsenstreif " and two lateral "Flimmerstreifen", its 

 structure, therefore being exactly comparable to that of the upper trefoil portion of the Actiniacean 

 filament. The processes arising from the filament were shown, however, to be very different in 

 structure from the Actiniacean acontia, being extremely attenuated folds of the filament, possessing 

 "Flimmerstreifen" as well as "Nesseldrüsenstreifen" and of each of these an ascendine as well 

 as a descending limb. 



Van Beneden confirmed the Hertwigs' observations as to the trefoil structure of the 

 upper portion of the filament and also von Heider's account of the greater extent of the trefoil 

 in the fertile than in the sterile mesenteries ; he noted too that the greatly coiled portion of 

 the filament occurred only on the sterile mesenteries and that the filament in this portion of 

 its course was no longer a trefoil, but simple. His most important addition to our knowledge 

 of the filaments was the discovery of true acontia in larval forms, arising from the free edge 

 of one or more of the fertile mesenteries, usually some distance below the termination of the 

 mesenterial filament and resembling closely in their structure the acontia of the Actiniaceae. 

 This discovery completed the proof of the distinctness of the branching processes of the trefoil 

 portion of the filaments and the acontia, and it consequently seems advisable that we should 

 now have a term to distinguish the processes. The Hertwigs, it is true, named them "Mesen- 

 terialfaden", but this term is a little too similar to mesenterial filament to prevent confusion, 

 and it would seem both convenient and apposite to translate the term and to speak of the 

 processes as craspedonemes. 



Owing to the excellent preservation of some of the examples of P. fimbriatus in the 

 present collection it has been possible to make a thorough study of the structure and arrange- 



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