XIII 



PHYLUM CHORDATA 



57 



not directly on the exterior, but into a cavity formed by the union 

 of paired ridges of the body- wall, and therefore lined by ectoderm. 

 The mouth gradually passes to the ventral surface, and under- 

 goes a relative diminution in size : a fold of integument develops 

 round it, and forms the oral hood, which is probably to be looked 

 upon as a stomodaeum. The endostyle appears on the right of 

 the pharynx (Fig. 709, fi), and is at first rod-shaped, then V-shaped : 

 ultimately the limbs of the V unite in the middle ventral line. 



t/ 



The gill-slits increase in number, and become more and more 

 vertically elongated. The provisional caudal fin disappears. The 

 gonads arise from the outer and ventral regions of the proto- 



FIG. 711. Amphioxus lanceolatus. Diagrammatic transverse sections of three larvae 

 to show the development of the atrium, ao. aorta ; c, derniis ; d, intestine ; /. fascia ; 

 /A, cavity for dorsal fin-ray ; m. myomere ; n. nerve-tube ; p, atrium ; sf. metapleural folds ; 

 si, sub-intestinal vein ; si; sheath of notochord and neuron ; */. sub-atrial ridge ; sp. coelome. 

 (From Korschelt and H eider, after Lankester and Willey.) 



vertebrae in the form of pouches, which gradually assume their 

 permanent form. , The development of the nephridia is not 

 known, but an organ, considered to be a provisional nephridium 

 (Fig. 709, #), is formed in the mesoderm of the first metamere, 

 and opens into the pharynx : it disappears in the adult. 



Distribution. Amphioxus has been found in the North 

 Atlantic and Mediterranean, on the west coast of North America, 

 the East Indies, the east coast of South America, Australia, 

 New Zealand, and the Malayan Islands. Asymmetron was first 

 known from the Bahamas, and a second closely allied species has 

 been found in the Louisiade Archipelago. As might be expected, 

 no fossil remains of the group are known. 



