V. 



THE PROTOCHORDATA IN THEIR RELATION 

 TO THE PROBLEM OF VERTEBRATE DE- 

 SCENT. 



" Den Schliissel richtigen Verstandnisses gibt nicht das Hineinpressen 

 nener Thatsachen in eine alte Sckablone, sondern das Anfsuchen des 

 genetischen Zusammenhangs der Erscheinungen. WEISMANN. 



BALANOGLOSSUS. 

 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. By 

 means of numerous unicellular integumentary glands which 

 are distributed over the surface of the body, 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 proboscis 

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

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

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



242 



