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



throe orders, ami except in the directive septa projects into the intrascptal space ; it is a 

 powerful muscular protuberance, which begins at nearly equal distances from the middle 

 point and tlir periphery of the pedal disk, becomes distended half-way up, and then gradu- 

 ally becomes narrower till it is inserted at the oral disk inside the tentacles (PI. VII. fig. 5). 

 The muscular protuberance lies almost freely on the surface of the septum, and is only 

 fastened to it near its adaxial margin by a kind of mesentery. In transverse section, it 

 therefore shows (fig. 3) a figure resembling a mushroom-shaped excrescence, a broad mass 

 from which a stalk thrust to one side passes up to the septum. The connective substance 

 of the septum enters through the stalk into the longitudinal muscle, where, seen in 

 transverse section, it becomes dendritically branched. The ramified lainelke of connective 

 tissue, which produce the dendritic figure in transverse section, are covered by a con- 

 tinuous layer of muscular fibrilke ; the whole is covered with epithelium, which reaches 

 to the bottom of the interstices between the layers of connective tissue, so that the 

 endodermal muscular fibres never become transformed into mesodermal fibres. 



The longitudinal muscle described above is part of the layer of longitudinal fibres, 

 which is slightly folded in other places, and shows in transverse section a second smaller 

 dendritic figure at the base of the septum only. Opposite it, on the other side of the 

 septum, we reach the site of the transverse muscular fibres, which are directed trans- 

 versely from the wall towards the axis of the body of the Actinia, and as usual are 

 strongest in the upper third. The parietobasilar muscle is found on the same side, where 

 it can be distinctly recognised as originating by a pleating of the transverse muscular 

 layer. It is, in fact, simply a crescentic fold lying loosely on the septum, so that a pouch- 

 shaped space opening into the stomach, into which one can thrust the point of a needle, 

 always runs in between the two parts. The fold is covered on both sides with an ample 

 muscular layer running parallel to the margin of the fold. The parietobasilar muscle 

 springs from the pedal disk, from its margin nearly to its middle, after which it is 

 attached to the wall as far up as the lower circular muscle. When the animal is 

 contracted it draws the pedal disk and the wall nearer one another, and as the former 

 is the part which is more easily moved, it becomes arched upwards, and so forms a slightly 

 depressed sucker ; the muscle therefore plays an important part in attaching the body 

 of the Actinia to the ground beneath. As regards septal stomata, the inner or peristomial 

 appear to be present, whilst the outer or marginal are certainly wanting. 



I consider the small Actinia described above as identical with a small form found by 

 Dana, near Valparaiso, in the American expedition under Captain Wilkes. According to 

 Dana's description, the whole animal is whitish, with a touch of yellowish-brown, the oral 

 disk pale flesh-colour, and the tentacles yellow. Drayton gave it the name of Actinia 

 ntjmphcea, which was afterwards changed by Milne-Edwards into Paractis nymphcea ; 

 finally, Verrill included the species with a mark of interrogation in the genus Sagartia, 

 for which, however, there is no sufficient ground. 



