REPORT ON THE ACTINIARIA. 45 



several points about it which seemed to indicate it as a suitable object for such a purpose. 

 In the first place the unusual size of the body is favourable to dissection by means of 

 knives and scissors, and in the second place, it was represented in the Challenger 

 material by a large number of tolerably well preserved specimens. Two of the indi- 

 viduals were in a state of intense contraction, whilst in the other three the tentacles still 

 projected through the opening formed by the upper margin of the half-contracted wall. 



The pedal disk is moderately thick, iiTegularly warted on the surface, otherwise 

 flat. It passes at right angles into the wall, of which the surface is perfectly smooth, 

 except in the upper part, which is folded longitudinally in consequence of the contraction 

 of the circular muscle. Most of the animals are distended like a drum, as sometimes 

 happens in the Actiniae, so that the wall has become a thin membrane with the 

 origins of the septa shining through it. At its upper margin only, where it is con- 

 nected with the oral disk, the wall becomes thickened to from four to five times its usual 

 strength (fig. 12), and shows in transverse section a yellowish tract, lying in whitish 

 fundamental tissue close under the endoderm, which is caused by the circular muscle 

 running in this part. 



The bundles of fibriUae appear in transverse section (PL VII. fig. 7) as roundish or 

 repeatedly indented figures, whose periphery consists of a corona of fine fibres, but whose 

 centre appears in the spirit material almost empty, whilst in the living animal it is 

 filled with protoplasm and the nuclei of the muscular corpuscles. The bundles of 

 fibrillee lie so closely together in the fibrous fundamental tissue of the mesoderm that it 

 is hardly possible to determine distinctly whether or not they are united into smaller 

 and larger groups. As the section shows, they become divided and united by anas- 

 tomoses into an annular plexus, lying parallel to the course of the fibrUlaj, i.e., parallel to 

 the pedal disk (fig. 9). 



Difierent points in the distribution of the bundles of fibrillse favour the view that the 

 mesodermal bundles originate in the endoderm, and only become deposited secondarily 

 by detachment in the mesoderm, where they increase still more by division and separa- 

 tion. The bundles of fibrillse lie usually in layers parallel to the endodermal surface, as 

 a few more compact layers of supporting substance extend through the mass of the 

 bundles parallel to the endoderm. The largest bundles are placed nearer the ectoderm, 

 where they are separated from one another by broader layers of connective substance, 

 whilst the smallest bundles (fig. 8) lie close under the endoderm, and — what is the 

 most important point — are connected here and there with the circular layer of fibres 

 which run on the endodermal surface of the mesoderm. 



The oral disk is covered with numerous shallow furrows, running from the oral margin 

 towards the tentacles. Their radial muscles form a tolerably broad stratum in the meso- 

 derm, and this is separated from the ectoderm by a thin, and from the endoderm by a 

 thick, layer of connective substance (PI. VII. figs. 10 and 11). This stratum is again com- 



