31. Muscles within the body wall force the air back and forth between the lungs and vocal sac. 32. The spines are essentially moveable extensions of the body wall , and are hollow and covered by cuticle. 33. These outgrowths of the body wall are distinguished from chaeta, which grow from follicles and thus possess roots. 34. The last two to three pairs of ribs are not connected and hang freely in the body wall . 35. The body wall consists of the epidermis and an external cuticle, which consists mainly of criss-cross collagen fibers. 36. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic. 37. Histolysis then occurs and the body wall becomes brittle and eventually bursts, liberating the eggs into the burrow. 38. The parietal pleura receives its blood supply from the intercostal arteries, which also supply the overlying body wall . 39. The main feature conssted of five arms, or ambulacra, in the body wall radiating outwards from the central mouth. 40. The gastrovascular cavity itself is divided into a number of chambers by mesenteries radiating inwards from the body wall .
