POSTLARVAL DEVELOPMENT. 6l 



in Astroides. In these the oral end was sometimes bifurcated and sometimes 

 the aboral. 



No trace of any calcareous deposit or skeletal formation was apparent 

 in any of the larvae before fixation, but calcareous matter was laid down soon 

 afterwards, the distinction between basal disc and free polypal wall, or skeleto- 

 genic and non-skeletogenic tissues, being then established for the first time. 



It is convenient to describe the general course of the development and 

 character of the young polyps before proceeding with the detailed growth of 

 the individual organs tentacles, mesenteries, and septa. 



YOUNG POLYP. 



Some of the lan'se became adherent to small pebbles, others to pieces of 

 glass, while most were fixed to the sides and bottom of the glass vessels in 

 which the colonies were placed. In this last case the vessels were sacrificed 

 in order to obtain the young polyps in a condition favorable for examination 

 under the microscope. By carefully breaking the glass suitable fragments 

 were secured with the polyps attached, and these could be transferred from 

 one aquarixim to another, or placed in dishes small enough to rest on the 

 stage of the microscope. In this way the growing polyps could be examined 

 at any time from both their upper and under aspects, their partial trans- 

 parency allowing the development of the mesenteries and septa to be followed 

 day by day. Thus the growth of the polyp and the skeleton could be observed 

 together and their various relationships studied. 



Once the larvae were settled they seemed vigorous and hardy, and scarcely 

 au}^ of the young polyps arising therefrom succumbed, although subjected 

 to the somewhat adverse conditions of small aquaria. 



The different organs, polypal and skeletal, began to appear shortly 

 after fixation, the various stages in the development of the tentacles, mesen- 

 teries, and septa being represented by the drawings and photographs on plates 

 1-5. The young polyps were early capable of expansion and retraction. 

 For the most part they remained in the expanded condition, with the tentacles 

 stretching out horizontally, or even overhanging. Sometimes the column 

 would extend vertically upwards and remain equal in diameter throughout ; 

 at other times the lower part of the walls would shrink over the corallum 

 and the remainder appear as a narrow column upon a broad pedestal. Upon 

 retraction the tentacles still remained exposed, but restricted to the central 

 region (plate 2). Even in the adult the tentacles have been found to remain 

 visible upon the fullest retraction, the column being incapable of folding 

 over them. On one occasion only a larval polyp was found with the 



