THE MUSCULAR SYSTEM 



263 



gives it iii side view the shape of a letter V. All alike are innervated by 

 somatic motor nerves. 



MYOTOME I 



MYOTOME 10 



ANTERIOR/; 

 CAVITIES 



ENDOSTYLE 



A. AMPHIOXUS EMBRYO, ist perm. gill- slit 



FACIALIS GANGLION 



OTIC CAPSULE 

 MYOTOME 4- 



MYOTOME 10 



SPIRACULAR POUCH 1ST GILL-SUT 



B. CYCLOSTOME EMBRYO. 



HYPOBRANCHIAL 

 MUSCLE 



OTIC CAPSULE LATERAL TRUNK MUSCLE 



MYOTOME 4 



IST GILL-SLIT HYPOBRANCHIAL MUSCLE 



C. ADULT CYCLOSTOME. 



Fig. 218. — 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. 



The visceral muscles connected with the intestine are non-metameric, 

 and are differentiated into an inner circular and an outer longitudinal 

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



