Osmotic Relationships in Opbeodesoma spectabilis— Freeman 
63 
Koizumi (1935), and the body fluids are iso- 
motic with the sea as has been found for most 
marine invertebrates (Prosser, 1961), then water 
should follow a gradient from outside to inside. 
To determine which structures are involved in 
passing water might provide insight into the 
mechanisms of water volume regulation. The 
most direct approach was to find the water path 
used by drained animals in restoring their body 
fluids. Draining animals is a simple but impor- 
tant technique which has some bearing on con- 
clusions to be drawn later; hence, it is described 
in some detail 
Integument 
with 
rrtueus 
Integument 
without 
mucus 
Opbeodesoma under natural conditions is in 
a turgid state with the body wall distended 
through retention of a large volume of coelomic 
fluid (Fig. 2 A). Out of water, the animal re- 
sembles an elongated balloon. If the animal 
is suspended by the head, the coelomic fluid 
flows to the tail region; the fragile integument 
stretches and usually ruptures, releasing the fluid. 
To induce animals to drain naturally through 
BD - Weight of animal before draining 
AD = Weight of animal after draining 
I = Weight of animal after 24 hours 
immersion in 100% sea water 
Fig. 3. Determination of functional site of water 
uptake in animals by use of plastic caps. 
D - Drained 
A - After 24 hours 
FIG. 4. The inability of animals with mucus-free 
integuments to restore coelomic fluid after 24 hr in 
100% sea water. 
the anus, they were supported in a horizontal 
position so that the body wall would not rupture 
and a 20 -gauge needle was inserted carefully 
into the anus. Gentle manipulation with the 
needle usually provided the stimulation neces- 
sary to release the sphincter muscle. The fluid 
then would drain, leaving the animal consider- 
ably reduced in size (Fig. 2 B) . Upon being re- 
turned to 100 % sea water, Opbeodesoma would 
immediately start restoring coelomic fluid, and 
by the end of 24 hr it would regain an amount 
close to that of its original volume. If rupture 
of the body wall did occur at any time in 
the course of an experiment, the animal was 
discarded. 
In the experiments animals were first weighed 
upon being removed from storage tanks. They 
were drained, reweighed, fitted with plastic caps 
in the various combinations indicated in Figure 
3, and then returned to sea water for 24 hr. 
Water intake evidently occurs only through the 
mouth, with the integument acting as a water- 
tight barrier. 
The importance of the mucus coating of the 
integument in making it impermeable to water 
was established by gently wiping off the mucus 
with a towel, draining the coelomic fluid from 
the animals, and returning them to sea water for 
24 hr. As is evident from Figure 4, animals so 
treated usually lose the ability to regain fluid. 
