PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 237 
4. Amphipod commensal 
One of the striking things associated with the external ap- 
pearance of Ascidia is the frequent presence of the young of a 
species of Orchestia within the cavity of the oral siphon. The 
occurrence of crustacean messmates in ascidians has long been 
the subject of comment, and many species of copepods have been 
described for ascidians all over the world (Scott, ’07). However, 
with the probable exception of Verril’s (’70) report of an amphipod 
in the ‘interior’ of Ascidia callosa, I have found no previous record 
of the free association of an amphipod in the branchial cavity 
of an ascidian. 
The species in A. atra is a pretty animal varying in size from 
two millimeters to nearly a centimeter. It possesses bright red 
eyes and a dark band across the middle of the back, both struc- 
tures showing conspicuously against the whiteness of the body. 
In the oral cavity of an Ascidia which has not been disturbed 
for a time, the amphipods are arranged near the rim of the siphon 
with the anterior end facing outward. Frequently as many as 
ten may be found in this position in a single siphon. It is a 
startling sight, when the blackness of the interior of the siphon 
is illumined, to see the brilliant red eyes of the creatures arranged 
in a circle a few millimeters within the cavity. 
The amphipods are capable of rapid locomotion when forced 
to leave their host, and may perhaps be free living at times. 
Their position in the oral siphon of Ascidia, however, is of dis- 
tinct advantage to them. The water current entering the oral 
siphon brings with it a host of small organisms to serve as food 
for Ascidia. The amphipods share this with their host, and, 
therefore, furnish an example of real commensalism. 
III. FOOD AND ENERGY SUPPLY 
The only source of metabolic and growth materials which is 
available to Ascidia is the surrounding seawater with its sus- 
pended and dissolved content. In order to utilize this supply, 
the animals perform certain activities whose function it is to 
furnish quantities of fresh seawater continuously, and to remove 
