16 THE VOYAGE OF H.M.S. CHALLENGER. 



septa, in which the remaining septa are grouped in pairs, but in which six is not the 

 fundamental number for the arrangement of the septa, e.g., Sicyonis crassa with sixty- 

 four pairs of septa, and Polyopis striata with sixteen pairs. The two directive septa 

 correspond to two distinct oesophageal grooves. 



In the second group we find two pairs of directive septa, the single septa are con- 

 structed precisely as in the true Actinias, but, with the exception of the directive septa, 

 are not united in pairs. I name Edwardsia as a type of this group. 



In the third division, to which only a single species of those examined by me, 

 Seytojihonts striatus, belongs, the variation from the typical condition runs in the 

 opposite direction. The paired arrangement of the septa is preserved, but one pair of 

 directive septa is wanting (PI. XIII. fig. 3). 



In the fourth division we can clearly recognise a single oesophageal groove on the 

 oesophagus ; the septa inserted at the bottom of the groove may also be defined 

 as directive septa, but it is not possible to point out on them the muscular arrangement 

 found elsewhere. They agree, however, with the other septa, in so far as they have a 

 thin layer of transverse muscular fibres on both sides. This is the case in Cerianthus. 

 The fifth division is formed by the Zoantkidas, in which the septa are paired, but partially 

 in a rudimentary condition. 



The plan which I have drawn up here, partly from the observations of others, but prin- 

 cipally from my own earlier and later investigations, of the structure of the Actinias, allows 

 me to make a few criticisms on the more important systems of Actinias already published. 

 Ehrenberg in his system of the Actinias, has made use first and foremost of the presence 

 or absence of the sucking papillae, then of the openings in the mural membrane, and, finally, 

 of the form, length, and arrangement of the tentacles. The sucking papillae recur in 

 Gosse's system, though they are made of subordinate importance ; they are described by 

 him as hollow papillae, furnished with a muscular apparatus, by which a vacuum is 

 formed. I have entirely omitted the sucking papillae in the general description of the 

 anatomy of the Actinias, as I have never observed them, even in forms which were 

 capable of incrusting themselves with foreign bodies. I am the more justified in doubting 

 their existence, as Gosse has given no proofs verifying his assertions. Jourdan has lately 

 described something like sucking papillae in Bunodes verrucosa, his " verrues glandu- 

 laires," epithelial cones, consisting almost entirely of glandular cells, which press into the 

 mesoderm, and partly form entirely or almost entirely detached mesodermal islands of cells. 

 Here, however, we must consider the fact that, in the case of an extremely j>apillose 

 surface, the depressions and sinuses between the papillas may often resemble glands 

 in transverse sections, taken through the wall. I have never been able to convince myself 

 of the existence of glands in Bunodes minuta, which does not, however, refute the asser- 

 tions as to their presence in Bunodes verrucosa. It is safer anyhow to consider the 

 adhesion of foreign bodies as brought about, on the whole, not by means of sucking-cups, 



