62 



ZOOLOGY 



SECT. 



The mouth gradually passes to the ventral surface, and undergoes 

 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. 763, fl), and is at first rod-shaped, then V-shaped : ultimately 

 the limbs of the V unite in the middle ventral line. 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 protovertebrae in the form of pouches, 

 which gradually assume their permanent form. 



Distribution. The Branchiostomidse are very widely dis- 

 tributed in tropical and warm-temperate seas. Amphioxides has 

 only been obtained with the tow-net and is, seemingly, of 



f 



FIG. 765. Amphioxus lanceolatus. Diagrammatic transverse sections of three larvse 

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

 (layer of connective-tissue on inner surface of myomere) \fh. cavity for dorsal fin-ray ; m. 

 myomere ; n. neuron ; p. atrium ; sf. metapleural folds ; sfh. metapleural lymph -space ; 



ridge ; sp. 



si. sub-intestinal vein ; sJc. sheath of notochord and neuron ; si. sub-atrial 



cojlome. (From Korschelt and Heider, after Lankester and Willey.) 



permanently pelagic habit. It differs from Amphioxus in the 

 absence of oral cirri and of an atrial cavity, the branchial slits 

 opening directly on the exterior in an unpaired ventral row. No 

 fully mature specimens have yet been found, but the larger speci- 

 mens have a single row of gonads. 



Distinctive Characters. The Acrania may be defined as 

 Euchorda in which the notochord extends to the anterior end of 

 the snout, in advance of the central nervous system. There is no 

 skull, and no trace of limbs. The ectoderm consists of a single 

 layer of cells which may be ciliated. The pharynx is of immense 



