V. 



THE PROTOCHORDATA IN THEIR RELATION 

 TO THE PROBLEM OF VERTEBRATE DE- 

 SCENT. 



■• Den Schliissel richiigen ]'erstandiiisses gibt iiiclit das Hineinpressen 

 neiier Thatsachen in cine alte Schablone, sondern das Aufsmhen des 

 genetischen Zusammenhangs der ErscheinnngenP — WElSM.iXX. 



BAL.\XOGLOSSUS. 

 External Features. 



Of the free-living protochordates, the lowest type of 

 organisation is undoubtedly presented by the Enteropneusta 

 (Hemichorda), the group to which Balanoglossus belongs. 



Balanoglossus is a remarkable worm-like creature which 

 lives buried in the sand or mud of the sea-shore. Bv 

 means of numerous unicellular integumentarv glands which 

 are distributed over the surface of the bodv, it secretes a 

 mucous substance to which particles of sand adhere, and 

 so makes for itself tubes of sand in which it lives at about 

 the level of the low tide-mark. It possesses such a 

 characteristic external form and odour (like iodoform) as to 

 render it peculiarly easy of recognition. 



In front there is a long and extremely sensitive //-('/v.Tr/V 

 which is capable of great contraction and extension, and is, 

 in the living animal, of a brilliant vellow or orange colour. 

 Behind the proboscis follows a well-marked collar-region, 



242 



