MUSCULAR SYSTEM 



179 



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

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



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 myocommas, between which 

 the fibres run longitudinally. The myomeres have an alternating 



Mo 



RM 



D 



Fi<;. 140. THE MUSCULATURE OF LARVAL AMKLYSTOMA (AXOLOTL). 



the side. 



From 



CV>, external ceratohyoid muscle ; Oph, cervical origin of the constrictor of the 

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

 muscles ; Dg, digastric ; Z>.s, dorsalis scapulre ; LI, lateral line ; Lf, latissiinus 

 dorsi ; Lr, levator arcuum branchialium ; ttt> levator branchiarum ; Ma, 

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

 the lateral muscles ; Mh\ mylohyoid (posterior portion) ; 0, 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 (Oh) ; at Re the oblique fibres of the latter pass 

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

 rcctus abdominis ; at Re 1 the rectus-system is seen passing to the visceral 

 skeleton ; J^h, 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 



