42 ZOOLOGY 



cavity or ccelom. This coelom consists of a series of ring-shaped 

 cavities, one in each somite, surrounding the gut, and separating 

 it from the skin. A rough model of the earthworm's body could 

 be made by taking a piece of gas-pipe to represent the gut, 

 and slipping over it a series of hollow rubber rings and then 

 enveloping the whole in a linen casing. The linen casing 

 would represent the skin, and the rubber rings the coelom. 

 As this model indicates, the coslom is not merely a series of 

 gaps intervening between other organs, but consists in reality 



coe 



Fig. 16. Diagrammatic cross-section of an Annelid 

 worm ; br, bristles ; coe, coelom or secondary 

 body-cavity ; g, gut ; A, blood-vessels repre- 

 senting the hgemoccele ; I, longitudinal muscles ; 

 o.c., outer circular muscles; p.p., parietal peri- 

 toneum ; v.m. , visceral muscles ; v .n., ventral 

 nerve cord; v.p. t visceral peritoneum. 



of a series of sacs, each with its own inner and outer wall. The 

 outer wall abuts on the skin, and is known as the parietal 

 peritoneum, whilst the inner wall is pressed against the gut 

 and is termed the visceral peritoneum. As the baby worm 

 grows, cells belonging to both walls become transformed into 

 muscular fibres. Those belonging to the parietal layer are 

 arranged longitudinally, and form the parietal or body 

 musculature, whilst those belonging to the visceral layer form 

 a series oi bands encircling the gut, and are termed visceral 

 muscles. In addition there are outer circular muscles derived 



