58 SIDERASTREA RADIANS. 



of the colony a score or so would be shot out together. Often when the disc 

 had become retracted a larva would remain within the upper peripheral region 

 of the polyp, fixed between the edges of a septum and the wall of a tentacle. 



The larvae were able to swim about immediately upon being discharged, 

 and gyrated through the water first in one direction and then in another. 

 There was evidence of a feeble negative geotropism. During the first day 

 they kept near the surface of the water, or gathered around the sides of the 

 vessels ; afterwards they traversed the water as a whole, though in the main 

 keeping near the surface. 



To the unaided eye some of the planulse appeared as minute, opaque- 

 white, spheroidal bodies ; others were oval ; but the majority were elongated 

 and pear-shaped, as shown in plate i , fig. i . Under the microscope the ciliation 

 was found to be uniform over the whole surface. The broader pole (oral) 

 was deeply pigmented and invariably posterior in swimming ; no oral aperture 

 could be detected at first, but extrusions of Zooxanthellae, yolk, and cell 

 debris took place later, showing that the mouth was already functional. 

 Under the microscope the brownish yellow of the broad end was seen to be 

 due to the presence of numerous Zooxanthellae within the ectodermal layer ; 

 otherwise the ectoderm was colorless, while the endoderm was dark and non- 

 transparent. The narrow anterior end (aboral) was perfectly colorless, its 

 ectoderm being free from commensal algae. In some larvae the colorless end 

 (aboral) was larger than the other. The elongated specimens were able to 

 retract and extend themselves, and upon irritation or preservation altered 

 their shape so as to become nearly spherical. The ordinary pear-shaped 

 individuals were about 2 mm. long. 



Throughout their free existence the majority of the larvse remained as 

 wholly opaque objects, and no trace of mesenterial divisions could be seen 

 from the outside. Some specimens, however, were so far developed as to be 

 distended at birth, or became so shortly afterwards. The tissues of these 

 latter were more transparent than the rest, and eight mesenterial divisions 

 were obvious, though not all of the same vertical length (plate i, figs. 3 and 4). 

 The latest extruded larvse, after the colonies had been kept under somewhat 

 unfavorable conditions for two or three weeks, were nearly devoid of external 

 Zooxanthellae and therefore colorless. 



After the first day or two many of the larvse showed positive geotropism 

 and sank to the bottom of the vessels, lying there motionless ; later, some of 

 these resumed their activities. By the evening of the second day a few had 

 fixed themselves by the narrow, anterior, colorless end to the bottom or side 

 of the vessel, or to objects placed within it. At first the larvse would adhere 



