CHAPTER XIII 
MUSCULAR SYSTEM 
LOCOMOTION——SOUND-PRODUCING ORGANS— 
ELECTRIC ORGANS : 
Muscular System.—The various muscles of the body may be 
arranged in two systems: (1.) the somatic or parietal, composed of 
striated or voluntary muscle-fibres; and (11.) 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 
