CNIDARIA 95 



Each pair of these ventral groups turns its macroseptum toward the 

 ventral pair of directive septa. Finally, in the Gerianthea (Fig. 43 D) 

 only one oesophageal groove [siphonoglyphe] is found. Here the numer- 

 ous septa are not arranged in pairs ; two particularly small septa attached 

 to the base of the oesophageal groove (A. von Heider) may be called direc- 

 tive septa. The septa lying at either side of them are the largest, and 

 from here the septa continually decrease in size toward the dorsal side, so 

 that it is probable that the zone of growth of new septa is situated at this 

 place (Hertwig). That the number of groups is possibly not concluded 

 with the types described, is proved by Gonactinia, which represents a 

 peculiar type allied to the Zoantheae (Blochmann und Hilger, No. 74). 



With respect to the development of the mesenterial fila- 

 ments, H. V. Wilson (No. 99) has proved, at least as far as 

 concerns the filaments of the twelve primary septa, that they 

 take their origin as outgrowths from the ectodermal epithe- 

 liam of the oesophagus. Even earlier A. vON Heider, on the 

 basis of histological agreement, had argued for the ectoder- 

 mal nature of the filaments in Cerianthus, and E. B. Wilson 

 had conjectured that at least the lateral ciliate bands (Flim- 



erstreifen) belong to the ectoderm. A. Andres also believed 

 that he had convinced himself that the filaments of the six 

 principal septa take their origin by means of outgrowths 

 from the ectoderm of the oesophagus. According to the 

 observations of H. V. Wilson on Manicina areolata, it is to 

 a certain extent probable that not only the lateral ciliate 

 bands, but also the nettle- and gland-cell bands {Nessel- 

 drmenstreifen), arise from the ectoderm. 



With respect to the more detailed processes of development, the mesen- 

 terial filaments of the first pair of septa differ from those appearing later. 

 The establishment of the first pair of septa and the filaments belonging 

 to it takes place in Manicina areolata at a time in which the space 

 between the oesophagus and the body-wall is still filled throughout by a 

 solid mass of entodermal cells. This cell-mass encircling the oesophagus 

 ^Kis divided into two parts, corresponding to the two primary gastral 

 ^^■>ouches, which are subsequently hollowed out. This division is effected 

 ^^py the formation between the oesophagus and the body-wall of two par- 

 ^^Kitions of the sustentative lamella, which constitute the foundation of the 

 ^^^nrst pair of septa. It takes place in this way : the oesophagus approaches 

 HHue body-wall until it comes in contact with it, then its sustentative 

 lamella fuses with that of the body-wall ; when subsequently the oeso- 



Iphagus again separates from the body-wall, a bridge of the sustentative 

 jlamella is preserved between the two. While the fundament of the first 

 ! 



