POSTLARVAL DEVELOPMENT. 65 



in Canada balsam as permanent preparations. The young coralla from 

 which the photographs on plates 4-5 were taken were prepared by macer- 

 ating the polyps in caustic potash and afterwards washing or carefully 

 brushing away the soft, loosened tissues. After maceration the skeletons 

 could generally be freed from the glass to which they were attached by 

 gently tapping the latter, but in some cases it was necessary to insert the 

 edge of a razor between the basal plate and the surface of the glass. 



The young polyps intended for sectionizing were detached after preser- 

 vation by dissolving away the calcareous deposit with weak hydrochloric 

 acid. In one case a living polyp of fourteen weeks became detached along 

 with the whole skeleton, and, thus free, continued its growth for some time. 



In the course of their growth the polyps increased from i to 2 mm. in 

 diameter, the oldest polyps reared being but 2 mm. across the base. 



TENTACLES. 

 FIRST CYCLE OF KXOTENTACLES. 



The tentacles began to make their appearance two or three days after 

 fixation of the larvae, as rounded upgrowths over alternate mesenterial 

 chambers. At this stage only the six primary pairs of mesenteries were 

 present, giving rise to six entoccelic and six exocoelic chambers (plate i, 

 fig. 7). Six tentacular prominences appeared simultaneously, equal in size 

 and distance apart. They early showed an opaque white, knob-like apex 

 distinct from a short, more transparent stem. The single cycle which they 

 constituted served to delimit the larva for the first time into oral disc and 

 column wall, no indication of the boundary between the two areas being 

 hitherto determinable. 



The stems of the tentacles were hollow, broad below, narrow distally, 

 and perfectly smooth, that is, without nematoblast tubercles. The endoderm 

 within could be seen to be richly supplied with Zooxanthellae, while the ecto- 

 derm was colorless ; the knob was solid and colorless, without Zooxanthellae, 

 and constituted wholly of ectoderm with numerous radiating nematoblasts. 



At first the actual mesenteric chambers from which the tentacles 

 protruded could not be determined with certainty, owing to the opacity of the 

 polyps. Soon, however, as the polyps grew larger and the walls more trans- 

 parent, the outgrowths were seen to communicate with the exocoelic chambers. 

 As noticed below, this relationship is contrary to the general rule of tentac- 

 ular development hitherto observed among the Zoantharia, and therefore 

 the greatest care was exercised to establish it beyond a doubt. Externally 

 the entoccelic and exoccelic mesenteric chambers could be readily determined 



