314 



THE DOGFISH. PHYLUM CHORDATA 



myomere (Fig. 237). The myomeres do not lie straight, but each 

 is bent five times, so that it runs a zigzag course from the middle 

 of the back to that of the belly. In the muscles of the head, 



mym. 



1 



viyc 



Fig. 237.— The hinder part of the trunk of a dogfish seen from the left side, 



with a piece of the skin removed. 

 /./., Tube of the lateral line ; wye, myocommata or septa of connective tissue ; mym., myomeres. 



throat, and fins the segmental arrangement is not apparent. 

 The myomeres are separated by partitions of connective tissue 

 (myocommata, singular myocomma), between which their fibres 



Fig. 238. — Successive positions of a swimming dogfish at intervals of o.i sec. 

 The lines are 3 in. apart. The passage of a wave is marked by dots. — From 

 Young, The Life of Vertebrates, 1950. Clarendon Press, Oxford. After Gray. 



run longitudinally. When the fish is swimming, waves of curvature 

 are produced by alternate contraction of the muscles of the two 

 sides (especially powerful in the tail, which is more than half the 

 length of the body). These waves travel from head to tail, and 

 -he leading, caudally-facing, surface is always directed obliquely 

 'rwards except when it is at its extreme displacement (frames 



