STRUCTURE OF AMPHIOXUS 



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nerve-cord there arise two sets of nerves, dorsal and ventral. 

 Of these the two anterior pairs of dorsal nerves, which go to 

 the pre-oral hood, are called cranial, and do not correspond 

 with the myotomes. Behind these a pair of dorsal nerves 

 arise at each myotome, but, as is the case with most other 

 segmentally arranged parts of the lancelet, the members 

 of a pair are not directly opposite to one another. The 



—Development of atrial chamber in Amphioxus. — 

 After Lankester and Willey. 



In I. the metapleural folds are seen sending a slight projection inwards. In II. 

 the two projections have united and enclose a small space (AT.), which is the 

 beginning of the atrial chamber. In III. this space is enlarging at the expense 

 of the coelom, which it pushes up before it. A comparison of this figure with 

 the cross-section of the adult (Fig. 296) will show the relation of coelom and 

 atrial chamber. 



FR., Coelomic space within dorsal fin ; AL., gut ; S., ccelomic space of metapleural 

 fold; MP., metapleural fold; SAT., projection which forms floor of atrial 

 chamber; ^O., aorta ; 5.C., coelom ; S./.F., sub-intestinal vein ; iV., nerve- 

 cord ; SH., sheath of notochord (NCH) ; MY., myotome; C, remains of 

 myocoel; AT., atrial chamber. The dotted line indicates the mesodermic wall 

 of the coelom. 



ventral nerves are absent in the region of the first two 

 pairs of dorsals, and behind this they divide up into many 

 minute fibres just as they leave the nerve-cord. The two 

 sets of nerves are compared respectively to the single-rooted 

 sensory dorsal nerves, and to the many-rooted motor ventral 

 nerves of higher Vertebrates. But the dorsal nerves of 

 Amphioxus supply the transverse muscles as well as the 

 skin, so that they must be partly motor. Furthermore, 



