REPORT ON THE ACTINIARIA. 69 



The constitution of the margins is the standard by which I have determined that the 



stomidia may he normal j >ln nomeiia and not merely rents in the oral disk. The outer 

 stomidia leading into the interseptaJ spaces are separated from one another by narrow 

 ridges, which have arisen from the septa, belonging to a pair converging upwards and 

 becoming directly united. The roof of an intraseptal space furnished by the oral disk, 

 which is usually of considerable breadth, has consequently undergone almost complete 

 retrograde formation. Towards the oral opening the stomidia are surrounded by swollen 

 lips, folded like frills ; these arc still more perceptible on the inner stomidia, round 

 which they form a border. 



As the arrangement of the stomidia follows that of the tentacles in other Actiniae, 

 there seems no doubt that they represent the latter morphologically. I have already 

 shown iu the Introduction that they may be derived in the most simple way from the 

 tentacles if we assume that the wall of the tentacle has become contracted into the 

 encircling lip-swelling, whilst the terminal opening has become proportionably widened. 



The oral disk is thickly pleated inwards from the stomidia, and covered with radial 

 swellings, which lie between the insertions of the septa and gradually disappear towards 

 the oral opening. The radial muscular fibres do not pass into the mesoderm, but remain 

 in the ectoderm ; like all muscular fibres of Polystomidium, they are very powerful, and are 

 united into a thickly-pleated lamella. The muscular folds are specially high in the 

 peripheral parts of the oral disk, where they lie thickly compacted and repeated! y 

 branched (figs. 4 and 7). 



On the oesophagus there is a remarkable circular fold, which runs at a little distance 

 below the labial margin, and marks off in this way a small upper section of the oesophagus. 

 Openings, equal in number to the stomidia on the oral disk, lie in this section, and lead 

 directly into the radial chambers (fig. 1). I have only observed similar formations in 

 Polyopis striata, another Actinia in which the tentacles have undergone retrograde 

 formation. The lower section of the oesophagus is covered with numerous longitudinal 

 furrows. Besides these there are two well-marked oesophageal grooves, and two long 

 oesophageal lappets, by which the directive septa can be easily determined. 



The number of the septa is smaller than in most of the larger Actiniae. Calculating 

 the number in the entire animal from the quadrant in which I dissected the septa, and 

 from the longitudinal lines on the surface of the body, there are altogether thirty-six pairs 

 of septa ; six pairs of principal septa of the first order, six pairs of the second order, and 

 twenty-four pah's of the third. The last number is very remarkable, as there are usually 

 only twelve pairs of septa of the third order. In consequence of this the interseptal 

 spaces of the second order are divided, not as usual into two, but into three interspaces, 

 because of the duplication of the septa of the third order. In this way Polystomidium 

 patens shows a variation from the regular conditions of the hexamerous A-Ctinise. 



The muscular part of the septa is very thick and powerful, and uniformly strong 



