THE MUSCULAR SYSTEM 



195 



The viscera] muscles in the wall of the intestine are non-metameric, 

 and are differentiated into an inner circular and an outer longitudinal 



MYOTOME I MYOTOME 10 



ANTERIOR/. 

 CAVITIES 



ENDOSTYLE' 



A.AMPHIOXUS EMBRYO. isT perm.gill-slit 



FACIALIS GANGLION 



LENS 



OTIC CAPSULE 

 MYOTOME 4- 



myotome 10 



SPIRACULAR POUCH 1ST GILL-SUT HYPOBRANCHIAL 



B. CYCLOSTOME EMBRYO. ^^^^""^ 



OTIC CAPSULE LATERAL TRUNK MUSCLE 



MYOTOME 4 



IST GILL -SLIT HYPOBRANCHIAL MUSCLE 



C. ADULT CYCLOSTOME. 



Fig. 184. — Diagrams illustrating the origin of the hypobranchial muscles of verte- 

 brates. Lacking in cephalochordates (Amphioxus), hypobranchial muscles make their 

 first appearance in cyclostomes in the form of muscle buds from post-branchial myo- 

 tomes. They become the tongue muscles of tetrapods and are innervated by the 

 hypoglossal (XII) nerve. In cyclostomes as in higher vertebrates myotomes i, 2, and 

 3 form eye muscles. 



layer. In the region of the gills, the visceral muscles are connected with 

 the gill cartilages, and are dififerentiated into levators, depressors, and 

 constrictors of the gills. 



