166 ZOOLOGY, 
ventral vessel. Stpunculus and Phascolosoma have remarkable 
bodies known as “urns” floating in their coelomic fluid. They 
are bell-shaped structures, with a ring of cilia round the mouth, 
anda nucleus. These remarkable corpuscles are formed by the 
division of certain large cells on the wall of the dorsal blood- 
vessel, they were formerly thought to be parasitic Infusoria. 
The Achaeta have no special organs of locomotion, and 
probably do not move about much. Sipunculus and Phas- 
colosoma usually live half embedded in the sand, which they 
swallow in large quantities. Phascolion lives in empty worms’ 
tubes or in molluse shells, and its body is often permanently 
twisted, accommodating its shape to that of its home. Phymo- 
soma lives in holes or passages in coral rock, or in holes 
between stones. Asa rule the members of this subdivision 
occur only in comparatively shallow water. 
The Gephyrea chaetifera are provided with a_ pro- 
stomium, which may acquire enormous proportions. In bonellia 
it may, when fully extended, attain a length of 2 or 3 feet, 
whilst the body is only 14 to 2 inches long. In this genus it 
is bifid at the end. In Hehiurus, Bonellia, and Thalassema 
there are a pair of large chitinoid hooks placed anteriorly on 
the ventral side of the body, and in some species of Echiwrus 
there is one, sometimes two, posterior circlets of setae, each 
seta originating from a single cell, like those of the Chaetopods. 
Bonellia viridis is coloured a bright green by a pigment 
termed “bonellein,’ which is not identical with chlorophyll. 
The mouth in the Chaetifera lies at the base of the pro- 
stomium, which is ciliated and grooved, and is doubtless used 
to catch minute organisms for food; the intestine is looped 
and the anus terminal. In Bonellia, Echiwrus, and Thalassena 
a “siphon” or collateral intestine, such as is found in the 
CAPITELLIDAE and ECHINIDS, is present. 
Branched organs open into the rectum in most of the 
Chaetifera. At the end of each branch is a small funnel- 
shaped ciliated opening leading into the coelom. The cells 
lining the tubes of these branches have been seen crowded 
with excretory granules, and they may possibly function as 
nephridia as well as serve to regulate the amount of fluid in 
the coelom. 
