5\) THE VOYAGE OF H.M.S. CHALLENGER. 



Dimensions. — Pedal disk, 2 - 5 and 1*5 cm. ; height, 1"2 and 0'5 cm. 



The two Actinia:, which I shall now briefly describe, belong to that class of specimens 

 in which the shape of the body has been so decidedly modified by the high grade of 

 contraction, and the colour is so completely gone from the action of the alcohol, that we 

 must observe very great caution in referring them to any species hitherto figured and de- 

 scribed. We must also bear in mind that in the case before us, even the larger specimen 

 under examination is not yet mature, and we must therefore consider that the structure 

 may undergo considerable changes in the course of growth. 



The pedal disk and wall are tough-walled ; they seem to have been perfectly smooth 

 in a fresh condition, and only to have become irregularly wrinkled and pleated in con- 

 sequence of being preserved. The wall is thickened two or three-fold for a short 

 space at the upper end by the circular muscle. The latter is separated from the 

 endoderm by a narrow layer of connective substance, and greatly resembles in form the 

 circular muscle of Tealidivm cingulatum figured in Plate VI. fig. 2. Seen in transverse 

 section, it widens towards the upper end like a club, though not so strikingly ; towards 

 the lower end it runs out into a fine point, by which it nearly reaches the endoderm. 

 The bundles of fibrillae are formed of a few very strong fibrillae, which are apposed one to 

 the other in form of a ring in transverse section ; they are separated by a sparse layer of 

 interstitial substance, and are only indistinctly arranged in larger and smaller groups. 

 The smallest bundles are found towards the lower pointed end, where they often merely 

 consist of from three to four fibrillae. 



The tentacles, whose number may be roughly estimated at about a hundred, are 

 placed in three circles, the innermost are the longest and decidedly the strongest ; 

 they measure more than - 6 cm., even in the contracted animal, whilst the outermost 

 present very thin filaments only - 3 cm. in length. I could not perceive any 

 terminal openings. The muscular system on the surface is a repeatedly folded layer 

 of ectodermal fibres, which also pass uniformly on to the oral disk. By this difference, 

 and also by the varying character of the circular muscle, Dysactis rhodora can be at 

 once distinguished from Dysactis crassicornis, in which the muscles of the oral disk and 

 the tentacles have passed into the mesoderm, whilst the circular muscle lies close under 

 the endoderm. 



Any description of the oesophagus would be of little interest. I shall therefore pass this 

 over, and proceed at once to discuss briefly the septa, the regularity of whose arrange- 

 ment is remarkably clear in section. There are in all four orders ; the six pairs of 

 principal septa and the six pairs of secondary septa are perfect, and only distinguishable 

 from one another by the former being more muscular than the latter. The septa 

 of the third order are imperfect and essentially smaller, whilst the last septa are 

 narrow, thin lamella?. In the quadrant, used for investigation, the septa of a cycle 



