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



and does reach the oesopliagus. The remaining twenty-seven pairs are distributed in the 

 space to the right and left of the oesophagus, so that thirteen pairs lie on the one side 

 and fourteen pairs on tlie other. Each pair consists of a larger perfect macroseptum, 

 and a smaller imperfect microseptum. 



If we term the side marked by the larger pair of directive septa, the ventral side, we 

 see that almost all the pairs of septa are placed in such a way that the larger, perfect 

 septa are directed ventrad, the smaller imperfect septa dorsad ; the two pairs of septa 

 next to the small dorsal pair of directive septa form the only exception to this rule as 

 their dorsal septa are the larger, their ventral septa the smaller. We can therefore 

 distinguish two systems of pairs of septa, a dorsal and a ventral. The result is the follow- 

 ing perfectly regular arrangement of the septa : as a rule the larger and smaller septa 

 alternate, but at three points two small septa lie between two large septa, viz., at the 

 dorsal end where the small pair of directive septa lie, and a little way further right and 

 left from the directive septa, where the dorsal and the ventral systems of septa are 

 mutually bounded by small septa. 



At the ventral end, on the other hand, we find three spaces between large septa, 

 in which the small septa are wanting, viz., the intraseptal space of the large directive 

 septa, and the two adjacent interseptal spaces. To explain this more clearly I give 

 formulae for the dorsal (l), and for the ventral side (2), showing the distribution of 

 the sep>ta. 



1. Dorsal side, ( &c. gk gk : kg kg k\k gk gk \ kg kg &c. 



2. Ventral side, ( &c. kg kg kg kg g \ g gk gk gk gk &c. 



In these formula the letter g indicates the large septa, k the small septa, the dotted 

 lines the boundaries between the dorsal and ventral systems, the black lines the position 

 of the sagittal axis, the underlining the directive septa. 



Three of the twenty-seven pairs of sejata are still imperfectly developed, and mucli 

 smaller than the others ; the two pairs lying to the right and left of the ventral dLrective 

 septa, and the extra pair of sej)ta which is only present on the one side. As I discovered 

 from other polyps of the same Zoantlms colony, the two pairs named at first are the 

 youngest in age. Their macrosepta resemble on the whole the small septa of the other 

 pairs ; they have no mesenteric filaments, and the uppermost section only reaches to the 

 oesophagus. 



I got an explanation of the manner and sequence in which the septa are developed 

 from examination of a small polyp, only a few millimetres high. It had forty-eight 

 septa in all ; exclusive of the directive septa, there were twenty-one on one side and 

 twenty-three on the other (PL XIV. fig. 6). In the region of the smaller directive septa, 

 the conditions were the same as in the developed polyps, but towards the ventral side the 



