5i8 



CEPHALOCHORDA 



from the atrial cavity. Other muscles occur in the region 

 of the mouth, and elsewhere. Most, if not all, of the fibres 

 are striated. 



Nervous system. — The dorsal nerve-cord is shorter than 



nch. 



—a.c.f. 



Fig. 296. — Transverse section through pharyngeal region of 

 Amphtoxus. — After Ray I.ankester. 



sp.c, Spinal cord ; nch., notochord, beneath which lie the two dorsal 

 aortae ; m., myotome ; a.c.f., atrio-coelomic funnel, opening 

 into sub-chordal coelom ; C, caecum ; G., a genital sac with 

 ova ; mp., metapleural fold ; air., atrial cavity ; Ph., pharynx, 

 with dorsal and ventral grooves, and bars between gill-slits. 

 Note in the primary bars and in the ventral groove the small 

 coelomic spaces. The ectoderm is dark throughout. 



the notochord, and has no definite brain. In the anterior 

 region, however, there is some difl'erentiation in minute 

 structure, and the central canal widens out to form the so- 

 called cerebral vesicle, which in the larva communicates 

 with the exterior by a pore (the neuropore). From the 



