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



The free margin of each septum is occupied by the mesenteric filament. The upper 

 section is tripartite, the lateral ciliated streaks still lying beside the median glandular 

 streak, whilst the lower section is simple, and formed merely of the glandular streak. 

 This last comports itself differently on the different septa ; in the septa of the first and 

 second orders it is disposed in meandrous curves and coiled into a thick mass, which, 

 in the septa of the first order, is visible beneath the margin of the oesophagus, whilst it is 

 covered by the latter in the septa of the second order ; in the other septa it appears 

 as a slightly waved border. The acontia common to all Sagartice, and of which one at 

 least is found in each septum — even in the small septa of the fourth order- — -arise at a 

 little distance from the lower end of the mesenteric filaments. In transverse section the 

 acontium shows a roundish figure flattened somewhat to an oblong (PL XII. fig. 10). 

 On one of the longer sides there is an indentation, the expression of a groove which runs 

 on the acontium as far as its point of attachment to the septum. Histologically we can 

 distinguish an axis of connective tissue and a cylindrical epithelium, containing numerous 

 nematocysts, especially on the side of the acontium remote from the groove. In 

 Calliactis p>ctrasitica, we had previously distinguished fine muscular fibres and a layer of 

 nerve fibres ; these doubtless exist in Cereus spinosus, but the specimen was not well 

 enough preserved to admit of their being made out plainly. 



If we open a Cereus spinosus by a longitudinal incision we can distinguish the different 

 pairs of septa, without further dissection, by the constitution of the reproductive organs 

 and of the mesenteric filaments (fig. 5). The pairs of septa of the first order (h 1 ) are 

 recognisable by the thick coils of mesenteric filaments, which spring from them below 

 the oesophagus, whilst the pairs of septa of the second order (Ii 2 ), the reproductive 

 organs and mesenteric filaments of which are usually completely covered by the oesopha- 

 gus, appear only as sharply defined lamellae with smooth margins ; in the septa of the 

 third order (h s ), in which the coils of filaments are wanting, the ends of the reproductive 

 organs project from beneath the lower margin of the oesophagus, whilst the pairs of septa 

 of the fourth order are so small as to be quite out of sight. 



Phellia, Gosse. 



Sagartidse with a rough, cuticular sheath, which is firmly attached to the epithelium, 

 and leaves the upper part of the wall free ; the latter is smooth and becomes inverted 

 during contraction ; cinclides not demonstrable anatomically ; tentacles small, and few 

 in number. 



The cuticular sheath, which we have already found in Cereus spinosus, is still 

 more strongly developed in a number of Actinia?, but is here confined at the same 

 time to one part of the wall, leaving the other part free. Close underneath the 

 corona of tentacles, the free part of the wall forms a broader or narrower ring, 



