102 SIDERASTREA RADIANS. 



d-e, as in fig. 54 ; a synapticular growth from the left limb of septum a-d 

 perforates the mesentery b, and a slight depression of the wall between the 

 mesenteries b and c is the first indication of an entocoelic skeletal ingrowth 

 abont to separate the two. 



The section from which fig. 57 was taken reveals important altera- 

 tions taking place. The second-cycle mesenteries d and e are both united 

 with the discal walls, and along with /are at this level not perforated by 

 any synapticular bars. The two peripheral limbs of septum a-d are now 

 becoming free from one another at their inner angle, and also from the central 

 radial portion ; both may be regarded as distinct exosepta a-b and c-d, though 

 formerly they appeared only as the bifurcations of the single exoseptum a-d. 

 For the first time an actual entoseptum now occurs in the entocoele of the 

 new pair of mesenteries b and c, and at its free extremity is on the point of 

 uniting with the synapticular process from the exoseptum a-b. The septum 

 b-c is clearly a new formation, appearing within the now widely bifurcated 

 peripheral extremity of the original septum a-d, or, better, between the two 

 exosepta a-b, c-d. 



The septal modifications suggested by fig. 57 are completed in the next 

 stage, fig. 58. The exoseptum c-d is now distinct, like septum a-b, from the 

 central radial part, while exoseptum a-b and entoseptum b-c are united with 

 one another towards their free edge by a synapticulum perforating the 

 mesentery b. 



The section represented by fig. 59 passes through the exsert edges 

 of the septa, so that the column wall forms the lower boundary while 

 the upper boundary passes through the tentacular region of the disc, four 

 tentacular thickenings being represented. The mesenteries extend uninter- 

 ruptedly from one wall to the other, and include the free septal edges within 

 their chambers. The mesentery c is still free at its inner end, while the 

 mesentery b is perforated by the synapticulum still joining septa a-b and b-c. 

 The central portion of the original exoseptum #-^has now nearly disappeared. 



The next stage, fig. 60, is from a section near the uppermost extremity 

 of the retracted polyp and its calice, so that now each septum and the body 

 wall surrounding it are becoming distinct. The mesenteries b and c both 

 extend across the two portions of the column wall, and the septa a-b, b-c, and 

 c-d are distinct from one another, the entoseptum b-c being as yet smaller 

 than the exosepta on each side of it. 



Omitting the synapticula as merely incidental structures the develop- 

 ment of the three septa a-b, b-c, and -afcan be diagraintnatically represented 

 as in fig. 12 (a-f) below, which shows also their relations to the mesenteries. 



