32 2 AN INTRODUCTION TO THE STUDY OF FOSSILS 



SUB-PHYLUM I, ADELOCHORDA 



This comprises but nine genera and some thirty species, the 

 best-known of which, Balanoglossus, burrows in the sand or mud 

 of the sea-bottom. In this form the mouth is at one end of the 

 body and the anal opening at the opposite end; immediately 

 dorsal to the mouth is a small, cord-like body correlated with 

 the notochord of the typical Chordata. This somewhat con- 

 cealed relationship to the Chordata suggested the name from 

 Greek adelos, not manifest, + chorde, a cord. These forms 

 show relationship also to the annehds, to the Phoronida of the 

 Molluscoidea and to the Echinodermata. Unknown in the fossil 

 state. 



SUB-PHYLUM 2, UROCHORDA 



The tunicates are degenerate Chordata, with the notochord 

 confined to the tail region (whence the name from Greek oiira, 

 a tail, + chorde, a cord). The adult body is inclosed in a tunic, 

 suggesting the common name. 



In all but one small order the tail with its included notochord 

 disappears in the adult, which is free-swimming (e.g. Doliolum) 

 or sessile {e.g. the majority of ascidians or sea-squirts). Many 

 in both of these groups form colonies by budding, a process of 

 growth characteristic of plants and of many of the lower classes 

 of invertebrates, but absent in all higher invertebrates and in 

 all the Chordata except these degenerate forms. Most of the 

 ascidians also show degeneration in the development of cellulose 

 within the skin. Cellulose is a protective and strengthening 

 substance characteristic of plants. Unknown in the fossil 

 state. 



SUB-PHYLUM 3, VERTEBRATA 



The notochord extends through the greater part of the elon- 

 gated body and persists throughout life (in Acrania) or gives 

 place (in Craniata) to a jointed vertebral column or backbone 

 (whence the name from Latin vertebra, a joint of the backbone). 



