CHAP. XVII PEOTOCHOEDATA 569 



reasons :— The word " Vertebrata " is so deeply established in the 

 literature that it is impossible to eradicate it. If all Vertebrata must 

 have true vertebrae, then not only must the groups classed as Proto- 

 chordata be ejected from the phylum, but also Cyclostomata, Elasmo- 

 branchii. Dipnoi, Chondrosteid Ganoids amongst fish, and the lower 

 Stegocephala amongst fossil Amphibia. If Amphioxus be "a-centrous" 

 so also is the sturgeon. We must not draw a line which wiU bifurcate 

 natural groups, hence the name "Vertebrata" must apply to the 

 whole group of animals characterized by dorsal tubular nervous 

 system, notochord, and giU-shts. 



HEMICHOEDA — ENTEEOPNEUSTA 



The Enteropneusta are represented at the present day by two 

 totally distinct types of animal, both of which are marine. One, the 

 Balanoglossida, resemble in outer appearance " worms," like Nemertea 

 or Annelida, and lead a burrowing life, inhabiting the soft mud at 

 the .bottom of shallow waters all over the tropical and temperate 

 regions of the world ; whilst the other type, the Cephalodiscida, are 

 sessile colonial animals, very much resembling Polyzoa in appearance 

 and habit of life, and confined to the colder temperate, arctic, and 

 antarctic regions of the sea. They were indeed, until very recently, 

 confounded with Polyzoa. 



About the embryology of the Cephalodiscida practically nothing 

 is known ; on one occasion when a fresh colony was dredged by a 

 vessel proceeding on a voyage of Antarctic exploration, it was seen to 

 emit oval ciliated larvae. The subsequent death of the naturalist 

 attached to this vessel has prevented this most interesting discovery 

 from being adequately followed up. 



Of the development of the Balanoglossida, however, we know a 

 great deal more. Long ago it was discovered by Metschnikoff (1870) 

 that a remarkable larva named Tornaria, which had been frequently 

 captured in the Plankton by the tow-net, and which had been mis- 

 taken for the larva of an Asteroid, was really the larva of a Balano- 

 glossid worm. Then Bateson (1884-1885) worked out completely 

 the hfe-history of a form, Balanoglossus (Dolichoglossus) Kowalevskii, 

 which has somewhat yolky eggs and a shortened larval development. 

 This development bears much the same relationship to the life-cycle 

 which includes the Tornaria larva as does the development of Asterina 

 gibhosa to that of Asterias with its bipinnaria larva. 



Bateson's work, which is the foundation of our accurate knowledge 

 of Enteropneust development, and which led to the view that the 

 Enteropneusta have vertebrate affinities, was challenged by Spengel 

 (1894), who questioned its accuracy and founded a view of the 

 relationship of the Enteropneusta with Annelida on his own observa- 

 tions on Tornaria larvae. It is therefore important to notice that 

 Bateson's conclusions have been confirmed, clarified, and reinforced 

 by a series of researches executed by American zoologists, of whom 



