2 ZOOLOGY SECT. 



may be enclosed in a firm sheath and thus be converted into a 

 stiff, but elastic, supporting structure. In the Craniata (with a few 

 exceptions among lower forms), it becomes in the adult replaced 

 more or less completely by a segmented bony or cartilaginous 

 axis— the spinal or vertebral column. Another nearly universal 

 common feature of the Chordata is the perforation of the wall 

 of the pharynx, either in the embryonic or larval condition only, 

 or throughout life, by a system of clefts — the branchial clefts ; 

 and a third characteristic is the almost universal presence at 

 all stages, or only in the larva, of a cavity or system of cavities, 

 the neuroccele, in the interior of the central nervous system. 



The Chordata are Coelomata (Vol. I., p. 340), and the -mode 

 of development of the coelome in the lower sub-phyla is essentially 

 the same as in the Echinodermata (Vol. I., p. 389), the Chsetognatha 

 (p. 318), and the Phoronida (p. 358) : it is derived, that is to say, 

 by direct outgrowth from the archenteron. In the Craniata this 

 enterocoelic origin of the cavity is no longer definitely traceable, 

 though what appear to be indications of it may be detected 

 in some cases. The Urochorda are not segmented ^ : in the 

 Hemichorda there is a division of the coelome into three parts, each 

 occupying a definite region of the body, so that the view is 

 sometimes maintained that these animals are tri-segmented : 

 in the Cephalochorda and Craniata there are numerous segments, 

 the nature of which will be referred to later. 



SUB-PHYLUM AND CLASS I.-HEMICHORDA (ADELOCHORDA). 



A number of worm-like, simply organised animals possessing 

 a structure which is commonly regarded as of the nature of a 

 rudimentary notochord, comprising Balanoglossus and certain allied 

 genera, are so widely removed from the other members of the 

 Chordata that, if we accept them as Chordates, it is advisable to 

 consider them as constituting an independent sub-phylum, and to 

 this the name of Hemichorda or Adelochorda has been applied. 

 Resembling Balanoglossus in the condition of the supposed noto- 

 chord, in the division of the body into three regions, sometimes 

 looked upon as representing three segments, and in certain other 

 features, are two genera of small marine animals — Cephalodiscus 

 and Bhabdopleura. These are probably more nearly related to 

 one another than they are to Balanoglossus, from which they are 

 separated by well-marked differences, and the Hemichorda may, 

 therefore, best be regarded as divisible into two classes— one, 

 the Unteropneusta, comprising only Balanoglossus ^ and its irame- 



1 Though faint indications of serial repetition of parts are traceable in certain 

 cases. 



2 The name Balanoglossus is here used as a general designation rather than as 

 a strictly generic term. 



