488 MAYOR. 



broader, rounded end being aboral and in advance during swimming, 

 while the oral opening is situated at the more pointed posterior end. 

 The alterations in form may be of two kinds; first, elongation, with 

 corresponding decrease in the radial axis, or shortening, with corre- 

 sponding increase in the radial axis ; or, secondly, a contraction of one 

 side of the planula so that the aboral end, which in such cases is 

 usually somewhat flattened, becomes turned to one side. In Sideras- 

 trea radians Duerden (:04) found the broad rounded end of the larva 

 to be the oral end. 



The elongated form is the one usually assumed by the larva when 

 swimming rapidly through the water. The larva takes a more defi- 

 nitely piriform shape when it swims slowly over the substratum. In 

 the latter case there may be a slight in-pushing of the aboral end form- 

 ing a hollow in the center. This hollowing out of the center suggests 

 a mechanism working by suction. When the larva comes to rest and 

 applies itself to the substratum, it becomes hemispherical, the aboral 

 end being the flat side of the hemisphere and applied to the substratum. 

 Later these larvae may become almost disk shaped. In the preserved 

 specimens the shape is usually either piriform or almost hemispherical. 

 After having become hemispherical and applied to the substratum, 

 the planula may detach itself, elongate and swim away. 



d. Fixation. 



The planula swims with the broader, rounded, aboral end foremost, 

 as already stated, rotating on its longitudinal axis as it does so. In 

 Siderastrea radians, which has the mouth at the broader end of its 

 piriform larva, Duerden (:04) found that, as is the case in Agaricia 

 fragilis, the aboral end is kept in front. From this it would seem that 

 while the form of the larva of Siderastraea is favorable for locomotion 

 that of Agaricia must satisfy other conditions than that of offering 

 the least resistance to forward motion. 



Larvae after they are expelled from the parent colony are usually 

 elongated in shape and swim through the water rapidly. Later they 

 become shorter and broader and swim more slowly. The normal 

 course seems to be for them to affix themselves to the substratum 

 within a few hours after they have been extruded. In the laboratory 

 larvae which did not become fixed in about twenty-four hours after 

 extrusion did not do so when kept for seven days, although during 

 this time they continued to swim through the water and also, as flat 



