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



of directive septa also belong to the six pairs of principal septa. These are easily found 

 as they lie at the opposite ends of the oral fissure, and thereby furnish us a fixed 

 point for the determination of directions. The remaining four pairs of principal septa 

 are distributed in the space in such a way that each two are found right and left from 

 the oral fissure at equal distances from one another and from the directive septa. The 

 six pairs of principal septa form together a regular six-rayed star. 



In an Actinia with the first six pairs or the first twelve septa, the space round the 

 cesophagus is divided into twelve radial chambers, of which six lie inside the pairs and six 

 between the adjacent pairs. The former are the " intraseptal " spaces or " inner" spaces, the 

 latter the " interseptal" spaces or " interspaces." Whilst the inner spaces remain unaltered, 

 the interspaces grow, and the accessory septa develop in them in pairs, and in an arrange- 

 ment which will not undergo any change. This definite arrangement may be shortly 

 characterised as follows : — A pair of septa lie in the middle of each interseptal space : 

 if we term the principal septa septa of the first order, or shortly, " primary septa," these 

 are the six pairs of septa of the second order, or " secondary " septa. They nearly equal the 

 primary septa in size, and, except in the Sagartidse, are fused with the oesophagus ; they 

 divide the interseptal spaces into three parts: (1) an intraseptal space of the second order, 

 and (2) two interseptal spaces of the second order. Then follow twelve pairs of septa of 

 the third order in the interspaces between the primary and secondary septa, twenty-four 

 pairs of septa of the fourth order in the interseptal spaces so formed, and so on. The 

 septa usually decrease in size, for whilst the first, which arise from the pedal disk and 

 from the wall, are inserted into the oral disk and the oesophagus, as far as the lower 

 margin of the latter, the succeeding pairs gradually extend to a less distance down the 

 cesophagus, then fail to reach it at all, and finally are attached only to the oral disk at a 

 distance from its centre. The same process is repeated at the pedal disk. The older septa 

 project nearly as far as the centre of the pedal disk, the younger only a little way inwards 

 from the periphery. As the size of the septa undergoes very gradual modification, we 

 can merely place them in two categories, " imperfect " sej)ta, which do not reach as far as 

 the oesophagus, and " perfect " septa, which are fastened to the oesophagus. After what 

 has been already said, it is unnecessary to add that all tlie pairs of secondary septa have 

 longitudinal muscles on the faces which are turned towards one another, and transverse 

 muscles on the faces which are turned away from one another. 



Methods of inquiry, difi'ering according to the size of the animal, are to be recommended 

 in order to recognise the above-mentioned conditions. Small s^jecimens may be examined 

 in transverse sections taken through the oesophagus, by which we survey the whole ar- 

 rangement of the septa at a single glance. Care must be taken, however, that the section 

 actually passes through the cesophagus and not somewhat through the oral disk, which in 

 contracted animals often reaches deep down into the interior. For example, it appears to 

 me not improbable that v. Heider gave too high a number of complete septa in Sagartia 



