XVIIl] 



DIVISIONS OF COELOM 



393 



The upper and anterior portion of the collar cavity becomes 

 separated from the rest : its inner walls thicken and develop into 

 a powerful longitudinal muscle which forms the first myotome 

 (Gr. /xv?, muscle ; ro/xos, a division). 



The "trunk coelomic cavity breaks up from the beginning into a 

 series of pouches called somites, each of which subsequently divides 

 into an upper and an under part. The inner walls of the upper 

 parts undergo a similar change to that experienced by the corre- 

 sponding part of the collar cavity, forming a series of myotomes. 

 The name myotome is given to each 'of the metamerically arranged 

 bundles of muscle-fibres. Each myotome is separated from the next 

 by a connective-tissue partition. In Amphioxus the myotomes of 

 the right side alternate with those of the left, so that the centre of 

 a myotome on one side is opposite the connective-tissue partition 

 on the other. Each is V-shaped, and they are arranged so <^{. 



o!f 



ci 



end tb m 



FIG. 188. 



Anterior region of young Amphioxus from left side, 

 the renal tubules inserted after Boveri. 



After Willey ; 



at. Atrial cavity, ci. Oral cirri, ch. Notochord. d.f. Dorsal fin-cli ambers. 

 e. Eye-spot. end. Endostyle. hep. Outgrowing liver ; the index 



line passes through one of J. Miiller's "renal papillae." met. Metapleural 

 fold. nph. Nephridia. nt. Spinal cord. olf. Olfactory pit. 



ph.b. Peripharyngeal ciliated band. tb. Tongue-bars. vel. Velum. 



Hence in a transverse section several myotomes are seen on each 

 side of the body. Thus we have two great series of longitudinal 

 muscles broken up into myotomes, one on each side of the animal, 

 by the alternate contraction of which powerful side-strokes of the 

 flat body propel the animal forwards. The elasticity of the notochord 

 acts like a fly-wheel in storing the force during the latter part of 

 each stroke and reinforcing each stroke at its commencement. 

 The cavity of the upper division of the somite persists throughout 

 life, and is known as the myocoel and the fold separating it from 



