8o 



SIDERASTREA RADIANS. 



the periods here given are for the most forward specimens. The six pairs 

 together constituted the second cycle of mesenteries or first cycle of metac- 

 nemes, the different pairs diminishing in size from the dorsal to the ventral 

 aspect of the polyp. Their order of appearance and comparative sizes are 

 diagrammatically shown in fig. 8 (d-f}. 



The interval between the appearance of the dorsal and middle, and 

 between the middle and ventral pairs, was usually well marked, so that at 



ii 



n 



IV 



ii 



VI 



VI 



VI 



II 



III 



Flo. 8 (a, I, c, d). Series of diagrams illustrating the order of development of the 

 first three cycles of mesenteries, a-c represent the order for the first cycle (protoc- 

 nemes), rf, the first stage of the second cycle (metacnemes). 



any time the different phases could be easily ob- 

 served. About four weeks elapsed between the 

 appearance of the first or dorsal pairs and that of 

 the third or ventral pairs. Usually, the pairs on 

 opposite sides of the polyp would arise simulta- 

 neously, but in one instance the left pair in the 

 ventral exoccele was apparent for over a week in 

 advance of the right pair in the corresponding 

 exocoele. In every instance the two members of a 

 unilateral pair appeared simultaneously and developed equally. 



The metacnemes increased slowly in length, and reached the aboral 

 termination of the column long before they extended to the distal extremity. 

 For many weeks the pairs differed in their vertical extent corresponding 

 with their dorso-ventral appearance ; even at the close of the observations the 

 distinctions were strongly indicated, giving to the polyp a bilateral symmetry 

 (plate 3, fig. 17). After the third month they began to extend across the 

 oral disc, the dorsal pairs being first recognizable, then the middle, and later 

 the ventral. They never, however, reached the stomodaeum, this condition 

 being also characteristic of the adult polyp. 



