EXTERNAL FORM. 21 



Similar groups of sensory cells occur on the margin of the 

 velum and its tentacles (Fig. 7). It may be noted, in 

 anticipation, that the velum is derived directly from the 

 mouth of the larva, which 

 becomes secondarily hid- 

 den from superficial view 

 by the overgrowth of the 

 oral hood. 



According; to Langer- IjlllKflS^^^^^PBii* ""^'^'^ 



HANS, similar cells to 

 those mentioned above, 

 carrying stiff sensory 

 hairs, are scattered dif- 

 fusely all over the exter- Fig. 7. — Velumof Amphioxusseenfrom 



nal surface of the body, ^'.^^^f '"' °' *'' p'"'^'"'- ^^'"'' ^^"""^^^ 



("Cf, Fio". 6 B.) But a i'..t/. Sphincter muscle of velum. z'.A Velar 



tentacles lying across the oral opening. 



concentration of sense- 

 organs comparable to the lateral line of the higher fishes 

 is apparently absent." 



A remarkable structure which seems to combine the 

 properties of gland and sense-organ occurs on the under 

 surface of the oral hood. It consists of a patch of 

 modified epithelium drawn out into finger-shaped epi- 

 thelial tracts, the cells of which carry long cilia. (See 

 Fig. 3.) It was discovered and accurately described by 

 Johannes Miiller, who called it the "Raderorgan" on ac- 

 count of the resemblance of its ciliary movements to those 

 of the wheel-apparatus of a Rotifer. The result of the 

 combined action of the cilia is to cause a flow of water 

 into the pharyn.x. In connection with the Raderorgan 

 must be mentioned a special depression forming a peculiar 

 sense-organ (Geschmacksorgan) lying against the right 

 side of the notochord, known as the groove of Hatschck. 



