CHAPTER XIII 



MUSCULAR SYSTEM— LOCOMOTION SOUND-PRODUCING ORGANS 



ELECTRIC ORGANS 



Muscular System. — The various muscles of the body may be 

 arranged in two systems : (i.) the somatic or parietal, composed of 

 striated or voluntary muscle-fibres ; and (ii.) the splanchnic or 

 visceral, consisting for the most part of unstriated or involuntary 

 fibres. Somatic muscles form ♦the great lateral longitudinal 

 muscles of the trunk and tail, which retain the primitive 

 embryonic metamerism to a greater extent in Fishes than in 

 any other Vertebrates, and are the principal muscles associated 

 with locomotion. The lateral muscles are composed of a series 

 of transverse muscle-segments or myotomes, which are >-shaped, 

 or S-shaped, or they even take a zigzag course from above 

 downward. The myotomes are disposed in pairs, and they are 

 separated from one another by fibrous septa or myocommata. 

 Each myotome is divided into a dorsal or epiaxial portion, and a 

 ventral or hypaxial portion, by a longitudinal, horizontal, fibrous 

 septum extending outwards from the vertebral centra to the skin. 

 The muscles of the pectoral and pelvic fins are derivatives from 

 more or fewer of the adjacent myotomes. The splanchnic muscles 

 include the musculature of the walls of the alimentary canal, as 

 well as those specialised portions of the visceral system which 

 are represented by the muscles of the branchial arches and the 

 jaws, and are composed of striated fibres. 



Locomotion.— A Fish and a Bird are equally remarkable for 

 the many and various ways in which they are adapted for locomo- 

 tion in the particular medium in which they live. In its shape 

 the Fish is admirably adapted for cleaving the water. Spindle- 

 like in shape, but thicker in front than behind, a Fish resembles 



349 



