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CEYLON PEARL OYSTER REPORT. 



mesenterial pair being situated within the angle formed by the bifurcated peripheral 

 extremities of the secondary septa. After a time a new cycle of entocoelic septa is 

 formed, each septum within a mesenterial pair of the second cycle, and as growth 

 proceeds these secondary entoccelic septa grow ceiitripetally, and fuse with inner 

 portions of the septa which originally constituted the exoccelic cycle. Thus a stage 



' X' 



1 x' 



j 2 3 



Fig. I. Diagram illustrating the development of the septa, and their relation to the mesenteries in Balano- 

 phyllia flmidana (Pouiitalks), and Siderastrwa radians (Duerden). The primary entoccelic septa are 

 shaded with cross lines; the exoccelic septa are dotted ; the secondary entoccelic septa are black; the 

 tertiary entoccelic septa are handed black and white. In fig. 1 the outer ends of the six exoccelic 

 'septa have bifurcated to form the secondary exoccelic septa A' 2 ; within each bifurcation a new 

 mesenterial cycle of the second order has been formed, embracing an entocoelic septum of the second 

 order En. 2 . In fig. 2 the secondary entoccelic septa have grown inwards and fused with the inner 

 limbs of the first formed exoccelic septa, thus forming the apparent second cycle of septa En? + A" 1 . 

 In fig. 3 the secondary exoccelic septa A"- have in turn bifurcated, and in each of the twelve 

 bifurcations a mesenterial pair of the third order is developed, embracing a tertiary entoccelic 

 septum En?. The primary entoccelic septa are denoted by 1, 1 in all the figures. 



is reached in which there are three cycles of septa ; two cycles, namely those which 

 are ordinarily reckoned as the first and second, are entoccelic, the remaining cycle, 

 which would ordinarily be reckoned as the third, is exoccelic. But it is clear that 

 each apparently secondary entoccelic septum is a compound structure, its inner end 

 formed of a septum which was actually second in order of appearance, its outer end of 

 a septum which was actually third in order of appearance. And it is further clear 

 that the exoccelic septa, which appear to constitute the third cycle, are really the 

 forked peripheral ends of the septa that were second in order of appearance. The 

 formation of the next cycle of mesenteries and septa follows the same rule. The 

 exoccelic septa become bifurcated at their extremities ; new mesenterial pairs of the 

 third cycle are formed within the bifurcations ; a new cycle of entoccelic septa is 

 formed within the pairs of third-cycle mesenteries, and these grow ceiitripetally and 

 unite with the inner limbs of the apparently third-cycle septa of the previous 



