600 J. E. DUERDEN. 
Parker! has determined the time interval between the moment 
of applying a stimulating agent (2°5 per cent. potassic chlor- 
ide) to the lips of Metridium and the occurrence of reversal, 
and also the interval between the application of pure sea- 
water and the return of the normal stroke. In five experi- 
ments the former varied from 30 secs. to 2 min. 30 seces., 
and the latter from 9 min. to 13 min. 10 sees. ‘The rapidity 
of reversal in Fungia when nutritive juices were employed 
was usually greater than in Metridium, taking place almost 
immediately upon application, but the return of the normal 
stroke varied much. 
In corals there must necessarily at times be incurrents set 
up independently of any external stimuli. An exhalent 
current, if continued, would tend to exhaust the fluid within 
the polypal cavity were there no incoming supply, and the 
coelenterate gullet has to serve as the passage for both the 
incoming and the outgoing streams. In this necessity for 
renewal we probably have an explanation of the fact that 
occasionally the non-nutritive particles of carmine, sepia, or 
even fine sand grains and fragments of shell were ingested, 
though usually they were rejected. ‘Torrey? has likewise 
found that in the actinian Sagartia davisi chemically inert 
objects like cork, paraffin, glass, and paper are at times 
swallowed, but he considers that these substances may cause 
a ciliary reversal by mechanical stimulation; Parker, how- 
ever, was unable to obtain any such reversals by mechanical ~ 
stimulation from inert materials. ‘The two opposite currents, 
inhalent and exhalent, proceed simultaneously in actinians 
like Metridium, which are provided with one or more 
siphonoglyphs, for Parker has demonstrated that the siphono- 
elyph current is always an inward one, whereas that over the 
lips may be either inwards or outwards*. As siphonoglyphs 
1 «The Reversal of Ciliary Movements in Metazoans,” ‘Amer. Journ. 
Physiol.,’ vol. xiii, 1905, p. 6. 
2 On the Habits and Reactions of Sagartia davisi,” ‘Biol. Bull.’ 
vol. vi, 1904, p. 212. 
3 Hickson (‘Phil. Trans.,’? 1883, p. 694) long ago found that in the 
