POSTLARVAL DEVELOPMENT. 59 



by means of the actual tip of the narrow end (plate i, fig. 5) ; this would then 

 flatten out, and the whole body become much shorter, a small round or oval 

 aperture being visible at the free extremity (plate i, fig. 6). 



Whether or not any individual larva would settle seemed very uncertain, 

 for out of several hundreds set free comparativel}^ few became permanently 

 fixed. Those which settled appeared to be the larger, better developed 

 specimens. Smaller vessels, glass slides, and cover glasses floated by 

 means of cork were placed within the receptacles so as to afford additional 

 surfaces for adhesion ; in other cases larvae were distributed within vessels 

 coated with parafiin. If fixation were not accomplished within the first few 

 days, it seemed to be impossible afterwards, though the larvae might continue 

 active for several weeks. In one instance about a score of planulae were 

 isolated and kept in a glass dish, and although they appeared healthy and 

 swam freely for a period of twenty days, externally the}' underwent no change 

 whatever. 



Sometimes a larva would fix itself in a position apart from others ; but in 

 general many would settle close together at one and the same time, their walls 

 in some cases actually pressing one against another. Plate i, fig. 5, represents 

 three larvae in the first stage of fixation, in this case to a small pebble. The 

 narrow extremities nearly toiich, and it is obvious that when the larvae shorten 

 and their bases flatten, the latter will exert a mutual pressure as a result 

 of their closeness. In plate i, fig. 6, seven larvae already settled and fully 

 expanded are represented, as seen from above by reflected light. All were 

 closely adherent to a fragment of stone, and formed a miniature colony. To 

 the under surface of a small pebble thirty-eight other specimens were aggre- 

 gated in groups of two, three, or more, the members of any group touching 

 along their margin. One of these groups contained a dozen or more 3'oung 

 polj'ps, all in contact with one another, the mutual pressure producing a 

 distortion of the normally circular base. 



During a single night another group of thirtj^-two became adherent to the 

 surface of a small glass dish. In this case nearly all the members were touch- 

 ing to a greater or less degree. One of the aggregations, consisting of fifteen 

 polyps, is represented in fig. 5, p. 60, as seen from the under surface of a 

 fragment of glass to which the individuals were attached. The colony when 

 drawn was two or three months old, and the skeleton was already in process 

 of development, being represented by a variable number of septa and by basal 

 and epithecal formations. 



The aggregated larvae, or young polyps, as they may be called after 

 settling, remained closely associated during their subsequent growth, and in 



