THE DEVELOPMENT OF ALCYONIUM DIGITATUM. 
58 
those which passed beyond the fifteenth to sixteenth free- 
swimming day without fixing eventually degenerated. 
The base of the newly fixed pear-shaped larva looks like a 
round pinkish disc from below, the planula for a time retaining 
its power of planarian-like contraction and expansion (Text- 
fig. 10). Soon the cilia 1 are retracted (Text-fig. 11), and the 
fixed polyp shortens first to a stumpy oval shape, and then to 
a flat mound shape by the end of the first twenty-four hours. 
At the beginning of the second day the mesenteries show by 
transmitted light as eight equidistant vertical ridges growing 
up from the base of the lateral walls (PI. 3, fig. 4, P.M.). 
These shallow ingrowths almost meet on the aboral and oral 
surfaces by the end of the second day (Text-fig. 12, d.), while 
eight small blunt conical outgrowths alternating with them 
en the oral surface indicate the tentacles (Text-fig. 13, and 
PI. 3, fig. 5,). On the third day the polyp has increased in 
size, and the stomodmum is formed by an invagination of the 
eral surface, within the circle of the tentacles (PI. 3, fig. 
6, M.). This tube deepens rapidly, and by degeneration of 
its base the coelenteron is put in communication with the 
exterior on the fourth day. Meanwhile the tentacles and 
body continue to lengthen (PI. 3, figs. 6 and 7) and soon the 
former develop two rows of pinnules, seven being the maximum 
number developed in the laboratory on any one row (Text- 
figs. 14 and 15). The polyps had on an average produced in 
each row : 
One pinnule by the sixth day. 
Two pinnules by the seventh day. 
Two to three pinules by the eighth day. 
Three to four pinnules by the tenth to twelfth day. 
Five pinnules by the twelfth day. 
Six pinnules by the eighteenth day. 
Seven pinnules by the twenty-first day. 
And while the older pinnules were carried along during 
development towards the tips of the tentacles, the new ones 
1 No cilia were seen on the ectoderm of the solitary polyp in any 
.subsequent stage. 
