THE EDWAEDSIA-STAGE OE LEBEUNIA. 273 



■strongly marked ridges and furrows, and even, in a few eases, 

 with as many as six or eiglit tentacles already developed. Only 

 the younger stages here continued to swim about; the older 

 straightway fixed themselves to the bottom of the vessel by 

 means of a flattened basal disc. 



Development of Tentacles. 



Within the first twenty-four hours the larvae of Lehrunia 

 settled down, and indications of the future tentacles were pre- 

 sented by a slight bulging from each of the eight intermesenterial 

 areas. These were at first rounded as in PI. 18. fig. 4 a, but, on 

 becoming larger, narrowed a little as in figs. 5 & 6. 



From the first a distinction into two alternating groups of 

 four was obvious, one series being slightly larger than the other, 

 though all constituted only one cycle. 



Many of the adult polyps are characterized by the occurrence 

 of superficial, opaque white areas on the oral aspect of the 

 tentacles, much more pronounced later on the columnar out- 

 growths. The opacity on the tentacles is found to appear at the 

 earliest stages, at first on the four larger prominences and later 

 on the four smaller. 



For some time it was not possible to recognize any difference 

 in size amongst the members of any one group of tentacles, the 

 disc presenting a perfect tetrameral radial symmetry. Later, 

 one tentacle became slightly larger than the rest. Following the 

 conventional nomenclature proposed by Kolliker and now usually 

 adopted, this will be the dorsal tentacle ; or, in the more morpho- 

 logical terminology of Haddon (1889), the sulcular. The tentacle 

 opposite will be the ventral, or sulcar, and the plaue in which 

 they are enclosed the dorso -ventral or sulculo-sulcar. In the 

 further increase in size the dorso-lateral pair of tentacles, that is 

 the tentacle on each side of the dorsal tentacle, remained a little 

 behind the ventro-lateral pair — these four forming the smaller 

 series in the earlier stage. 



The larva thus changed from its primary condition of radial 

 symmetry to a decided bilateral form, the sagittal plane passing 

 through the large dorsal and the ventral tentacles. 



At the expiration of two or three days the larvae had reached 

 the stage represented by fig. 7 (PL 18). The aboral end is now 

 flattened and disc-like, and even slightly larger in diameter than 



