68 



CERIANTHARIA. 



As we shall see, the position and structure of these craspedonemes in C. anicricanus throws 

 some light on the real nature of the organs which van Beneden has called acontia and made homo- 

 logous with the acontia of Actiniaria. 



"Acontia". 



Van Beneden has described as acontia certain thread-shaped, single or slightly branched 

 organs at the aboral end of various Ceriantharian larval forms. Van Beneden gives the following 

 account of the structure of the acontia in general. Around the central mesogloeal axis of the thread 

 spreads an entodermal longitudinal musculature. The epithelium that covers the mesogloea, contains 

 principally nematocysts and gland cells in large numbers, and in addition to these, supporting cells. 

 He has in some cases observed that the acontia do not issue from the border of the mesentery, but 

 that their attachments are somewhat lateral. This fact, as also the circumstance that the nematocysts 

 are so numerous in the acontia, tells, he thinks, in favour of the view that the acontia ave practi- 

 cally stinging batteries and that they are homologous with the acontia of Actiniaria. At the same 

 time he finds a few, though trifling differences in the acontia of Ceriantharia, namely (p. 31) "forme 

 de 1'axe mesenchymatique, uniformite de structure de 1'epithelium sur tout le pourtour de 1'axe, carac- 

 teres de 1'epithelium". In point of origin van Beneden considers the acontia to be endodermal 

 formations in distinction from the ectodermal filaments, a view which is supported by the fact that 

 the large nematocysts found in the acontia are exactly like those which occur in the entoderm of the 

 mesenteries. Lastly he holds the acontia to be homologous with the botrucnidae. 



These views of the structure and homology of the acontia were probably formed by van 

 Beneden in consequence of an imperfect examination of the acontia. As the bent form of the acontia 

 makes it often difficult to prepare satisfactory transverse sections from any point in the acontium's 

 length, I surmise that van Beneden's cross sections were as a rule taken from the distal portion 

 of the acontium only, whilst of the proximal portion they were usually longitudinal or oblique. 

 And in fact the study of sections so taken really does suggest such a theory of the structure of the 

 acontia. But a closer investigation of the acontia shews in my opinion that, in the matter of the 

 structure and homology of the acontia, van Beneden has been mistaken in several important points. 

 The acontia are practically a continuation and differentiation of the filaments and consequently - 

 assuming as is probable that the filaments are ectodermal are themselves ectodermal; only a part 

 of the acontia, that, namely, in which the large thick-walled nematocysts are numerous, is endodermal. 



I have examined "acontia" both in the Ceriantharian larval forms Arachnactis lobiancoi and 

 albida and in the Ceriantharium adult forms Arachnanthus oligopodus and sarsi. The acontia of Arach- 

 nactis albida, Arachnanthus oligopodus and sarsi correspond almost exactly in structure, whilst the 

 acontia of A. lobiancoi look more like the acontia of Ovactis, a larval genus described by van Be ne den, 

 in that they contain very numerous nematocysts in the entoderm part, whilst these are infrequent in 

 the other species examined. Before I describe the structure of the acontia, I wish to lay stress on the 

 fact, that the formations I have examined are the identical ones to which van Beneden applied the 

 name. In as much as van Beneden examined in the first place the threadlike processes at the 

 aboral pole of the longest mesenteries of Arachnactis albida and designated them as acontia, it is quite 



