610 J. E. DUERDEN. 
produced only on mechanical irritation of the surface of the 
polyp against solid particles. 
It is uncertain how far the exudation of mucus in corals 
is a continuous physiological process under ordinary cir- 
cumstances or to what extent it is dependent upon stimuli. 
It can scarcely be imagined that the falling of light débris 
upon the disc is sufficient to influence the exudation, yet once 
the particles reach the disc they become embedded in such a 
secretion, and are not removable except along with the 
mucus. It is probable, as in the case of so many aquatic 
animals, that the polypal surface is always provided with a 
thin mucous coating, and that from time to time the outer 
stratum sloughs off and is then renewed. In addition, there 
can be no question that mechanical stimuli increase the 
amount of mucus exuded; this is certainly the case on 
handling or irritating specimens, and may also take place 
when any but the lightest objects are dropped upon the dise. 
Nutritive solutions cause an increase in the amount of 
mucus exuded in corals to such a degree that it may envelop 
the entire disc and tentacles of the polyps concerned ; after 
a supply of meat juices as many as five or six mucous streams 
may be seen passing down the gullet together. The amount 
exuded for the time being is, however, limited in quantity, 
and can be wholly ingested, as may be inferred from the fact 
that the streams cease after a time, and a strand undergoing 
ingestion at both ends ultimately breaks in the middle. The 
exudation can be renewed by the application of a further 
nutritive solution, but experiments are still needed to deter- 
mine how long successive discharges will continue, or how 
voluminous they will remain. Parker! found that potassic 
chloride, which is capable of reversing the ciliary activity in 
Metridium, is also a stimulant for the production of mucus 
and the discharge of nettle capsules, apparently more so 
than meat-juice itself. 
The mucus undergoes some physical or chemical change 
1 <The Reversal of Ciliary Movements in Metazoans,” ‘Amer. Journ. 
Physiol.,’ vol. xiii, 1905. p. 4. 
