MUSCULAR SYSTEM 



179 



integumentary spines (e.g. Echidna) ; or may cause local movements 

 (" twitching ") of the skin (many Mammals). 



I 



Parietal Muscles. 



A. Muscles of the Trunk. 



In Amphioxus the body muscles are made up of a series (60 or 

 more) of lateral muscular segments or myomercs separated by 

 > -shaped connective-tissue septa or vnyocmnmas, between which 

 the fibres run longitudinally. The myomeres have an alternating 



FIG. 140. THE MUSCULATURE OF LARVAL AMBLYSTOMA (AXOLOTL). 



the side. 



From 



Ce. , external ceratohyoid muscle ; Cph, cervical origin of the constrictor of the 

 pharynx ; Cu, cucullaris ; D, dorsal, and V, ventral portion, of caudal 

 muscles ; Dg, digastric ; Z>,s, dorsalis scapulae ; LI, lateral line ; Lt, latissimus 

 dorsi ; Lr, levator arcuum branchialium ; ttf, levator branchiarum ; Ma, 

 masseter ; Me, myocommas between the myomeres of the dorsal portion of 

 the lateral muscles ; J\Ih\ mylohyoid (posterior portion) ; O, superficial 

 layer of the external oblique muscle, arising from the lateral line, and ex- 

 tending to the fascia, F; at * a piece of this layer is removed, exposing the 

 deeper layer of this muscle (Ob) ; at Re the oblique fibres of the latter pass 

 into longitudinal fibres, indicating the beginning of the differentiation of a 

 rectus abdominis ; at Re 1 the rectus-s3 r stem is seen passing to the visceral 

 skeleton : /Vi, procoraco-humeralis ; RM, dorsal portion of lateral muscles of 

 the trunk ; SS, suprascapula ; T, temporal muscle ; Th, thymus. 



arrangement on the two sides. On the ventral region of the 

 anterior two-thirds of the body is a thin transverse sheet of fibres. 



In Fishes the myomeres and myocommas, arising exclusively 

 from the mesodermic somites (p. 9), have a zigzag arrangement on 



N 2 



