12 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
become somewhat contracted and bent inward, so that they lie 
against a circumanal region, pentagonal in shape and without 
spicules. All previous observers have, I believe, overlooked these 
papillae. 
3. HABITAT, HABITS, FOOD, ETC. 
Caudina arenata lives buried in the sand where the water is at 
least a few feet deep at low tide. Though I have repeatedly tried 
to obtain specimens by digging at the water’s edge at the lowest 
spring tides, I have never succeeded in finding any in this way. 
During an easterly gale, if it be of sufficiently long duration, numer¬ 
ous specimens are likely to be dislodged from the sand and cast 
upon the beach. 
The variety of C. arenata described by Theel as var. armata was 
found to occur at depths of 898 fathoms and 1242 fathoms; in the 
former case at Lat. 35° 44' 40" N., Long. 74° 40' 20" W. (1880), 
and in the latter instance at Lat. 41° 24'45" 1ST. , Long. 65° 35' 30" W. 
Caudina burrows head foremost mainly by means of its tentacles, 
which by the alternate contraction of their outer and inner longi¬ 
tudinal muscles move back and forth in a radial direction, crowding 
aside the grains of sand which lie in its course. A forward move¬ 
ment is facilitated by the animal swallowing the sand immediately 
in front of it, as is said to be the case also with Synapta and many 
worms. The dark-colored organic matter contained in the sand, 
which is swallowed in great quantities, forms the principal food of 
the animal. 
As observed in an aquarium containing well-aerated sea-water and 
provided with an abundance of sand, so that the environment is as 
nearly as possible normal, Caudina lies for days at a time entirely 
buried in the sand, save the tip of the tail. The exposure of the 
tail permits the respiratory trees to perform continually their function ; 
through the anus, water is forced out of the respiratory trees and 
drawn into them by the alternate contraction and relaxation of 
these organs and of the wall of the body. The latter, by reason of 
its natural rigidity, resumes its normal shape when its circular 
muscles are relaxed, and so increases the capacity of the body-cavity, 
thus bringing about an influx of water. These movements are 
accompanied by the correlated opening and closing of the anus by 
