_Ctenodontidae_ (p. 505). _Uronemidae_ (p. 507). Ceratodontidae (p. 507). Lepidosirenidae (p. 511). OF UNCERTAIN POSITION Order. Family. –––––– ––––––– _PALAEOSPONDYLIDAE_ (p. 521). _OSTRACODERMI_ (p. 522) _HETEROSTRACI_ (p. 524) _Coelolepidae_ (p. 524). _Drepanaspidae_ (p. 525). _Psammosteidae_ (p. 526). _Pteraspidae_ (p. 527). _OSTEOSTRACI_ (p. 527) _Ateleaspidae_ (p. 528). _Cephalaspidae_ (p. 528). _Tremataspidae_ (p. 530). _ANASPIDA_ (p. 531) _Birkeniidae_ (p. 531). _ANTIARCHI_ (p. 532) _Asterolepidae_ (p. 534). _ARTHRODIRA_ (p. 535) _Coccosteidae_ (p. 536). HEMICHORDATA BY SIDNEY F. HARMER, Sc.D., F.R.S. Fellow of King's College, Cambridge. {3}CHAPTER I HEMICHORDATA CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—_CEPHALODISCUS_ AND _RHABDOPLEURA_—PHORONIDEA—_PHORONIS_ AND ACTINOTROCHA—AFFINITIES OF THE HEMICHORDATA. The Hemichordata, a marine group which includes the worm-like Balanoglossus, owe much of their interest to the fact that they are believed by many zoologists to be related to the lower Vertebrates. This view is one of a number of mutually exclusive hypotheses, which seek to derive Vertebrate animals from various Invertebrate ancestors. It is supported by many striking resemblances between Balanoglossus and the lowest forms which are by common consent regarded as belonging to the Vertebrate alliance; but it must be distinctly understood that Balanoglossus is at most the much modified modern representative of extinct forms which were also the ancestors of Vertebrates.