REPORT ON THE ACTINIARTA. 11 



to a varying extent above tlie surface of the septum, and shows many variations in the 

 details of its constitution ; a second specialised but much weaker cord stretches along 

 the wall, close to the origin of the septum. As the retractor in transverse section is 

 placed on the septum hke a pennon, Schneider and Eotteken have given it the name of 

 "muscular pennon." On the other hand, the " parietobasilar " muscle is differentiated 

 from the transverse muscles ; it lies in the angle between the pedal disk and wall, into 

 which it projects with a crescentic margin, like the plica semilunaris in the corner of the 

 eye. It extends to different distances up the wall and towards the central point of the 

 pedal disk. It originates from the transverse muscular layer, by a process of pleating which 

 is beautifully shown in Leiotealia nymphcea. As the muscular fold here still lies loosely on 

 the septum, we can pass a needle into the pouch-like interspace. Apart from the parieto- 

 basilar muscle, the transverse muscles are most strongly developed in the upper third of 

 the body of the Actinia (PL II. fig. 6 ; PI. IV. fig. 9 ; PL VII. figs. 5 and 12). 



As the two surfaces of a septum differ from and are unequal to one another in the 

 arrangement of the muscles, there are predispositions to a peculiar arrangement of the 

 septa which, with few exceptions, is found in all Actiniae, viz., that the septa are united 

 in pairs, so that we cannot speak appropriately of single septa but of pairs of septa. The 

 equivalent sides of the septa of the pair, i.e., the sides in which the muscles run in the 

 same direction, are turned towards each other. As a rule, it is the sides with the longi- 

 tudinal muscles, and only in two pairs the sides with the transverse muscles. These two 

 pairs of septa occupy a perfectly fixed position in the body of the Actinia, and may conse- 

 quently be used for fixing direction, on whch account we shall name them the " directive " 

 septa. The directive septa correspond to the oesophageal grooves, and are fastened to the 

 oesophagus from the oral angle downwards to the end of the lappets of the oesophagus. 

 They constitute the principal reason why such stress should be laid on the form of the 

 mouth, and they themselves contribute very essentially to a more clear expression of the 

 biradially symmetrical character of the body of the Actinia. 



The history of the development of the septa will help us to understand some further 

 characteristics of their arrangement. We see from it that the septa of a pair have 

 generally a common origin, and that only the first six pairs form an exception to this rule. 

 The most recent researches show that the septa of the first six pairs appear independently 

 and at different times, and that they become united secondarily in paii-s ; as they are 

 placed first, and according to a special principle, it is appropriate to distinguish them as 

 "principal" septa from the others, the " accessory" septa. In some cases (in Sagartia, 

 Phellia, &c.) they are permanently recognisable from the fact that they only are inserted 

 into the oesophagus ; usually, however, this peculiarity is shared by numerous accessory 

 septa, and they are then merely distinguished by their somewhat larger size. The 

 difference of size may, however, be almost equalised, which makes the determination of 

 the principal septa difiicult. This is, however, made easier by the fact that the two pairs 



