.REPORT ON THE ACTIXIARIA. 115 



septa all became smaller as they approached the larger directive septa ; on one side only 

 the first seven, on the other side only the first six, macrosepta reached the oesophagus, tin n 

 followed five other macrosepta which still decreased in size, so that the smallest, which 

 came next the ventral directive septa, hardly projected at all into the gastric space. The 

 microsepta left off still earlier, for they became smaller in exact proportion to the macro- 

 septa, and as they were in general less they disappeared sooner. On the ventral side the 

 directive septa only were perfect, and were separated by a wide interspace from the septa 

 which were next in development. 



Two facts may be deduced from the above observations : (1) The macrosepta and 

 microsepta can be distinguished from the first by the difference in size ; they develop 

 independently and at different periods, whilst in the Hexactinise and Paractinise the septa 

 of a pair start simultaneously and are of the same size from the first. (2) The septa are 

 not produced regularly in the periphery of the body of the Actinia, but within a limited, 

 ventral productive zone. The dorsal septa are therefore the oldest, the ventral septa the 

 youngest, with the exception of the directive septa, which are developed very early. 



The third polyp was intermediate between the two specimens described, both in 

 the size and the number of its septa, which amounted to fifty-two. A more minute 

 description of it is therefore unnecessary, and I shall conclude my remarks on the 

 Zoanthese with some details as to the structure of the septa. 



A cellular cord, or a canal filled with cells, rims in the supporting lamella of the septa 

 in immediate proximity to the wall (PI. XIV. fig. 2). It is usually divided into several 

 cords by commissures of the supporting lamella and is of such strength that the whole 

 septum becomes visibly and locally thickened. I never could make out any connection 

 between this septal canal and the ectodermal cords of the wall in any of the numerous sec- 

 tions which I prepared, and I am inclined to believe that it is produced from the endoderm. 

 My reason is that I have observed that the same roundish bodies which are to be found 

 in the endoderm, which I regard as parasitic, unicellular organisms, force their w r ay into 

 the septal canal, but never into the canals of the wall. I attach less importance to the 

 origin of the canals, as they seem to be connected with the endoderm here and there where 

 the septa spring from the wall. However, the figures, which led me to consider such a 

 mode of connection as probable, did not furnish sufficient proof of its actual existence. 



The structure of the mesenteric filaments is essentially the same as in the Actiniae ; dur- 

 ing the greater part of their course they consist entirely of the median glandular streak, 

 and it is only a little way below the oesophagus that they are widened by the addition of 

 paired ciliated streaks, the surface of which is indented at regular intervals by transverse 

 furrows. This upper section of the mesenteric filaments appears to me identical with the 

 "flattened organs" described by Yerrill as "having a curved or crescent form and a 

 transversely striated surface, — attached to the principal radiating lamellae, near the base 

 of the stomach" (Trans. Connect. Acad., vol. i. p. 491). Yerrill, Andres, and others 



