CERIANTHARIA. 69 



out of the question that I can have had before me any other formations than van Beneden's acontia. 1 ) 

 Nor do I think it probable that van Be lie den meant to denote by the term "acontia" threads of 

 unlike structure, that is to say, that van Beneden's acontia are heterogeneous formations -- but 

 that is a point which cannot be definitely cleared up until van Beneden's Ceriantharian larval 

 forms have been examined afresh. 



In Arachnantlius oligopodus, as I shew in another work (Mittheilungen z. Stat. Neapel 1912) 

 the acontia are in the form of a thread, which in transverse section is oval in shape. The craspedion 

 region is continued in the acontium, forming a stronger ascending limb and a slighter descending one. 

 In the most distal portion of the thread the two limbs of the filament run so close together, that no 

 boundary can be detected. Towards the base the descending limb gets thinner, so that on both sides 

 of it an endoderm part becomes visible and divides one limb from the other. At the base of the 

 "acontium" in Arachnanthus oligopodus the descending limb has disappeared, so that the thread con- 

 sists here merely of an ectodermal part, the ascending limb of the filament, and an endoderm part 

 going with it 



In Arachnactis albida the descending limb lasts longer on the free border of the mesenterial 

 thread, so that both limbs are met with at the base of the "acontium". The histological structure of 

 the acontium filament is rather different from that of the craspedion region. Supporting cells occur, 

 but the great bulk of the ectoderm is made up of mucus cells, and there is found only a thin 

 sprinkling of spirocysts and thick-walled nematocysts. The longitudinal muscles which are restricted 

 solely to the ectodermal part, are found in the distal portion of the acontium, where the fila- 

 ments enclose the whole mesogloeal surface, to run in a continuous layer all round the mesogloea: 

 on the other hand towards the base of the acontium the longitudinal muscles are differentiated into 

 two groups, one for each limb of the filament, or else (in A. oligopodus] only one group is found at 

 the descending limb. The mesogloea in A. oligopodus shews signs of breaking up about each limb 

 of the filament. The endoderm cells are almost exclusively supporting cells, only very rarely do we 

 find a large thick-walled nematocyst. In Arachnanthus oligopodus, sarsi and Arachnactis albida, all 

 whose acontia agree in structure, the function of the "acontium" therefore is not that of a sting 

 battery. 



Regarding the structure of the acontia in Arachnactis lobiancoi, the account I have elsewhere 

 (1912) given on another page is not so complete as that of Arachnactis albida and A raclmanthus oligo- 

 podns, as the sections of the basal portion were taken lengthwise, of the distal portion transversely 

 or obliqxiely, still I have obtained transverse sections approaching so nearly the basal portion that there 

 also the nature of the acontia is made clear. For, in the first place, it plainly appears that the "acon- 

 tium" consists of an ascending and a descending limb of the filaments, containing principally gland 

 cells. In the second place I have succeeded in ascertaining the important fact, that the large nema- 



i) Me. Murrich (1910) seems in contrast to Cerfontaine (1909, p. 686-87) to regard the "prolongements filatiien- 

 teux'' in Arachnanthus oligopodus as acontia. At the same time Me. Murrich, who has examined a number of larval forms 

 with acontia, states that he has nothing to add to the description of the acontia given by van Beneden. Cerfontaine's 

 description of these organs however, which according to my investigations fully agree with van Beneden's acontia in 

 Arachnactis a/bida, differ materially from van Beneden's description. Cerfontaine's account of the "prolongements fila- 

 menteux" in A. oligopodus agrees substantially with mine of the acontia. 



