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



Angelo Andres (Quart. Jour. Micros. Sci., new ser., vol. xvii. p. 221, 1877) and my 

 brother and myself (Actinien, p. 127) bad already pointed out that the position of the 

 septa in the Zoanthese was regulated on an entirely different principle from that in other 

 Actiniaria, though G. v. Koch was the first to find out the true nature of it. He dis- 

 covered what I fully corroborate, that the sejita present are of two different sizes (Morphol. 

 Jahrb., Bd. vi. p. 359, 1880). The larger or macrosepta only reach the oesophagus and 

 bear reproductive organs and mesenteric filaments, whilst the smaller or microsepta are 

 sterile and end on the oral disk ; the latter are not, as I formerly supposed, young septa 

 destined to be developed into larger, but are really rudimentary formations. 



Both the larger and the smaller septa bear muscles on both sides : one side bears 

 longitudinal fibres, the layer of which is, however, only slightly pleated, the other side 

 bears fibres which rise obliquely, and are homologous with the transverse muscular fibres 

 of the other Actiniae, though they can easily be mistaken for longitudinal fibres in trans- 

 verse sections. There is here, therefore, a predisposition to the paired arrangement of the 

 septa, the existence of which was first recognised by G. v. Koch. Each pair consists of 

 a small and a large septum, having longitudinal muscles on the faces turned towards one 

 another (PI. XIV. fig. 2). The two pairs of directive septa form an exception, however, 

 as one pair of them, the ventral, contains large septa only, — the other pair, the dorsal, 

 only small septa ; in some Zoanthese, we must also except two pairs of ordinary septa 

 which lie right and left at a little distance from the small directive septa, and contain 

 macrosepta only (PL XIV. fig. 3). 



The manner in which the larger and smaller septa are distributed can be more 

 accurately determined if we start from the directive septa, and disregard provisionally 

 the grouping in pairs. In the Zoanthese, as in all Actiniae, two kinds of septa 

 alternate ; in the septa of the one system the muscles are disposed in the same way as 

 they are in the ventral pair of directive septa, whilst in the other system the case is 

 reversed, and they have the same disposition as they have in the dorsal pair of 

 directive septa. The septa which have the same arrangement of the muscles as the small 

 dorsal directive septa, viz., the dorsal septa, are likewise small, whilst the others, the 

 ventral septa, are strong ; it is only in the neighbourhood of the small directive septa 

 that the conditions are reversed as the dorsal septa are strong, and the ventral septa are 

 weak. We can therefore divide the ordinary pairs of septa into two different regions ; 

 in the one (the larger, or ventral region), the ventral septa of the single pairs are macro- 

 septa, and the dorsal septa are microsepta, whilst in the other (the dorsal region), the re- 

 verse is the case, and the dorsal septa are macrosepta. When all the pairs of septa are 

 equally developed, the two regions are bounded on either side by microsepta, but those 

 two microsepta are often wanting left and right, and in this way the pairs formed of 

 macrosepta alone, which have been already mentioned, are produced : these contain two 

 large dorsal septa of the dorsal region and two large ventral septa of the ventral region. 



