AND HABITS OF THE AMMOCHARID. 237 
enough to see both kinds in action, and note that the coelomic 
fluid (in which sometimes ova were floating) almost invariably 
passed steadily through the sphincter valves when travelling 
towards the anus, and rapidly past the large slit or “ flap” 
valve when going in the opposite direction. In the absence of 
assistance from circular muscles, it will be seen that the latter 
form of valve is specially well adapted for rapid release of the 
imprisoned coelomic fluid, and that the animal is enabled speedily 
to retract the posterior portion of its body, in case of danger, by 
means of its longitudinal muscles. My observations show that,. 
with the exception of the buccal and “ thoracic” segments, and 
possibly the smaller ones towards the anal extremity, each side 
of each segment is provided with two valves, one of each kind. 
Those attached to the septum at the base of the “thoracic ” 
segment are described by Gilson (27. p. 384), and are excep- 
tionally powerful. As will be seen later, this is apparently 
another adaptation connected with the habits of the animal. 
The foregoing will probably suffice as a general description of 
the structure of the worms, and we may next consider their 
habits. They are exceptionally bardy animals, whose welfare is 
apparently more dependent upon a good supply of rather muddy 
sand than upon a liberal allowance of oxygen, since they thrive 
under conditions so various as those which prevail respectively 
on the British shores at low-water mark, and at a depth of 2975 
fathoms near the Caribbean Islands (18. p. 410); whilst both 
Dr. Fauvel and I have found them to live for years in small’ 
aquaria destitute of alga and without change of either sea-water 
or sand, the only attention they received being an occasional 
raking of the surface and the addition of river-water to eompen- 
sate for evaporation. They are gregarious, and dwell embedded 
near the surface of the sand in flexible sand-covered tubes. The 
tube is unique in structure, and, when fully extended, frequently 
two or three times as long as its inmate. It is well described 
by Grube (2), in 1846, as a transparent gelatinous tube, the 
middie part of which is supported with great firmness by a crust 
of grains of sand and fragments of shell which, when flat, are 
““olued ” on by their highest edges so as to overlie one another 
like the tiles on a roof. 
Gilson (20. p. 822) has given a most careful description of the 
minute structure of the tube, and although I am unable entirely 
to confirm his speculations on the mode of its formation, it is 
