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



their way into the thin margin surrounding the opening in the tentacle, but terminate 

 abruptly before they reach this point. The margin, therefore, merely consists of a 

 lamella of connective substance, covered by two layers of epithelium passing into one 

 another at the free edge. 



On the oesophagus (fig. 4), the two oesophageal grooves at once strike the eye as deeply 

 incised furrows, bounded by broad folds and running zig-zag, as secondary transverse 

 folds project alternately left and right. The other longitudinal furrows, which run, ten 

 in number, on either side between the oesophageal grooves, are less distinct. From the 

 oral disk the oesophagus is separated by a thick lip-like swelling, divided into twenty-four 

 parts corresponding to the number of the longitudinal furrows of the oesophagus. 



The number of the pairs of septa amounts in all to sixty-four. Sixteen of these are 

 of equal size and are inserted into the oesophagus ; alternating with these we find 

 sixteen other pairs only a little smaller, which end on the oral disk, but like the others 

 are purely muscular septa. On the other hand the last thirty-two pairs of septa, which 

 are equally distributed in the interspaces between the muscular septa, bear only the 

 reproductive organs and are furnished merely with a very thin muscular layer. There is 

 a very pronounced difference in size between the smallest muscular septa and the 

 reproductive septa, such as I have already described in Ojyhiodiscus. In Sicyonis also 

 the septa are merely thin-walled mesenteries for the reproductive organs, thick masses of 

 which (testes) occupy the free margin of the fold ; they only extend upwards to one- 

 third the height of the animal, and are entirely wanting in the angle between the wall 

 and the oral disk (fig. 9). 



Fig. 9 shows the distribution of the muscles on the muscular septa. On the side 

 of the longitudinal muscles a single cord radiates like a fan towards the oesophagus and 

 the central parts of the oral disk ; on the side of the transverse muscles the parietobasilar 

 muscle extends half way up the wall, where it occasions the constriction already 

 mentioned. In the perfect septa a small opening lies in the neighbourhood of the mouth. 

 The muscular septa and genital septa are finally to be distinguished by the fact that 

 the former only bear mesenteric filaments. 



Sicyonis crassa has another character in common with Opkiodiscus besides the 

 differentiation of the septa into reproductive and muscular,.viz., the relation in which 

 the number of the pairs of septa stands to the number of the tentacles. 



In the majority of Actiniae there are at least twice as many tentacles as there 

 are pairs of septa, so that each intraseptal and each interseptal space has its own 

 special tentacle. In Sicyonis and Opkiodiscus there is an equal number of pairs of septa 

 and of tentacles ; the thirty-two intraseptal spaces of the muscular septa only have their 

 own special tentacles, whilst the other tentacles belong in common to the thirty-two 

 intraseptal spaces of the reproductive septa and the sixty-four adjacent interseptal 

 spaces. This also shows the rudimentary character of the reproductive septa, since they 



