188 ANNULATA. 
a pait of ofocysts. These are situated laterally on the peris- 
tomium and are supplied by nerves from the 
brain. They consist of spherical sacs communi- 
cating with the exterior by fine ducts. The cells lining the 
Sensory. 
Fig. 114.—SECTION THROUGH THE OTOCysT OF ARENICOLA. 
(After ASHWORTH and GAMBLE.) 
External Aperture. 
Leta Rap 
ial 
Epithelium. 
sac are sensory and the cavity contains a number of loose 
concretions or ofo/iths which appear to be sand-grains. 
On the prostomium is a ciliated pit called the nuchal- 
organ, a probable sense-organ allied to the paired ciliated 
pits of Polygordius. Arenicola has no prostomial tentacles, 
but the prostomium is produced into two odes which are 
also probably sensory. 
The muscular system is well developed and consists of 
a circular layer under the ectoderm and a longi- 
tudinal layer inside it. There are also diagonal 
fibres running from the lateral lines to the mid-ventral line. 
Nearly all the animals usually called ‘‘ worms ” move by the system 
of circular and longitudinal muscles. The body of the ‘‘ worm” which 
contains coelomic fluid acts much like an elongated bladder or sausage- 
skin filled with fluid. When the circular muscles contract they press on 
the coelomic fluid which forces out each end and hence elongates the 
worm, reducing its calibre. When the longitudinal muscles contract 
the body is shortened, the coelomic fluid at the same time forcing the 
walls outwards. Hence the alternate movements of the two muscular 
Muscular. 
