POSTLARVAL DEVELOPMENT. 63 



containing Zooxauthellae, would at times be seen whirling round and round 

 within the stomodseum, and would afterwards be jerked out and lost to the 

 polyp. Even after three months such extrusions would still take place. 



The young polyps varied much in their rate of growth, especially in 

 the later stages ; indeed, in some instances, development seemed to be wholly 

 arrested. The isolated polyps progressed most rapidly, but among these 

 many differences were recognizable. The polyps forming colonies made 

 scarcely any progress after the first week or two. In the group represented 

 in plate i, fig. 6, no increase beyond the six tentacles and twelve simple 

 septa occurred up to six weeks, when the colony was preserved. 



The development of the various organs was characterized by certain 

 well-marked intervals of rest and growth. A week or two might pass without 

 any conspicuous change taking place, and then 

 progress would be somewhat rapid. The order of 

 appearance of the organs varied somewhat in dif- 

 ferent individuals ; at first some organs of a svstem 

 were developed a cycle at a time, and the remainder 

 in successive pairs from one border of the polyp to 

 the other. Perhaps under the artificial conditions 



the duration of some of the intervals was partly py "7" . T!7 *\ -*' 



determined by the state of the water and the amount 



.. , r1r1 i" i f -11 i Fie. 6. Diagram showing the course of the 



and character of the food supplied. Still, there circulation within the imemaj cavity of 



were certain definite resting stages appearing in all 



the polyps and seeming to possess a phylogenetic as well as an outogenetic 



significance. 



The following were the most conspicuous phases of growth : Of the 

 mesenteries the protocnemes were established at the time of settling of 

 the larvae, and the six pairs are shown to develop in a regular consecutive 

 manner without any marked intervals (p. 76). Four of the six bilateral 

 pairs of protocnemes were connected with the stomodseum from the commence- 

 ment of stationary life, but at the termination of seventeen weeks the fifth 

 and sixth protocnemic pairs had net become complete. No further change 

 in the number of mesenteries took place for nearty four weeks, when, in 

 the larger polyps, the metacnemes began to appear. The different pairs of 

 these followed somewhat quickly upon one another from the dorsal to the 

 ventral aspect of the polyp, until the complete cycle of six pairs was estab- 

 lished (plate 3, fig. 14). The polyps remained at this mesenterial stage 

 for the rest of the time they were under observation about two months. 

 There was no hint of the appearance of the third cycle. 



