346 G. H. PARKER 
to measure 3.2cm. Before contraction its volume was 27 cc. and 
after contraction this had been reduced to 3.2 ec. through the 
discharge of 23.8 cc. of water—a reduction of over 88 per cent of 
its volume. This very considerable change of form and volume 
was early observed by Miller (64, p. 353), who pointed out its. 
importance in specific descriptions in this genus. The complete 
contraction of a fully expanded Renilla may be accomplished 
under special stimulation in a minute or so. Its expansion, which 
necessitates that it shall refill itself with water, requires at least 
half an hour. 
Under natural conditions probably the majority of Renillas 
expand and contract as already indicated with the flowing and 
ebbing of the tide. Specimens kept in sand-filled aquaria in the 
laboratory, however, even though continually under water, will 
at times contract, bury themselves in the sand, and remain thus 
hidden for considerable periods. In one instance a Renilla re- 
treated under the sand and remained quiescently there for three 
and a half days, whereupon it was dug out and placed in sea- 
water. It then inflated itself and acted in other respects entirely 
normally. It is therefore possible for Renilla to remain con- 
tracted and quiescent for considerable periods, though this is 
probably not often the case under natural conditions. 
INCURRENT AND EXCURRENT APERTURES 
By what apertures water enters and leaves the body of Renilla 
is by no means certain. Even the number of kinds of openings 
in the body of this animal is still in dispute. At most four sets 
of such apertures have been distinguished. There are, first, the 
autozoéids, or ordinary polyps, generally scattered over the 
superior surface o! the rachis. Each of these zodids is provided 
with a mouth, which Agassiz (’50, p. 209) regarded as the chief 
means of entrance and exit of water for the colony as a whole. 
Next there are found among the autozodids and imbedded in 
small masses of whitish materials groups of pores each one of 
which represents the mouth of a lateral siphonozoédid. These 
siphonozooids were originally described by Verrill (64 b, p. 12) 
as rudimentary individuals. According to Wilson (’83, p. 725), 
