ANNUL AT A. 



a pair of otocysts. These are situated laterally on the peris- 

 tomium and are supplied by nerves from the 



Sensory. 



brain. They consist of spherical sacs communi- 

 cating with the exterior by fine ducts. The cells lining the 



Fig. 114. — Section through the Otocyst of Arenicola. 



(After AsHwoRTH and Gamble.) 



External Aperture. 



L Epithelium. 



sac are sensory and the cavity contams a number of loose 

 concretions or otoliths 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 lobes 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 



