656 



APPENDIX B 



There are about 70,000 existing species of Chordata, of which all but a rela- 

 tively small number are vertebrates. Four subphyla are commonly recognized, 

 but the first three of these may be combined under the name Protochordata. The 

 remaining subphylum, the Vertebrata, may be conveniently divided into eight 

 classes, though more than eight are often made. 



Subphylum A. The Primitive Chordates (Protochordata). This sub- 

 phylum includes the lower chordates, for the most part unrecognizable 

 as such except on the basis of their embryology. They possess, in some 

 stage, what are believed to be the homologues of notochord, gill slits, and 

 hollow dorsal nerve cord; but they lack a brain case, vertebral column, 

 paired appendages, true ventral heart, and hemoglobin in the blood. There 

 are three classes: 



Fig. B.26. Models of the primitive chordate Amphioxus (Branchiostoma). Upper, side view 

 showing the external appearance; lower, the left half mostly removed to show internal 

 structures. Note the perforated walls of the large pharynx. (Courtesy American Museum of 

 Natural History.) 



Class 1. The Acorn Worms (Hemichordata). Wormlike marine animals with a 

 muscular proboscis and collarlike neck region; without typical notochord, that 

 structure possibly represented by a short blind tube extending forward into the 

 proboscis from the dorsal wall of the alimentary canal; paired lateral gill slits 

 present. Some students of the chordates would exclude this group from the 

 phylum. Example: Balanoglossus. 



Class 2. The Sea Squirts, or Tunicates (Urochordata). Solitary or colonial 

 marine animals (Fig. 27.5) with a saclike covering called the tunic. Notochord 

 absent in adult, present in the tail of the tadpolelike larva. Adult either fixed 

 (sessile) or in a few forms free-swimming and pelagic. 



Class 3. The Lancelets (Cephalochordata). Amphioxus, a small fishlike marine 

 animal, with elongated body in which metamerism is evidenced by the V-shaped 

 muscle segments of the trunk. Notochord well developed, extending practically 

 the full length of the body. Gill slits numerous. Without true head or brain. A 

 good swimmer but spending most of time buried in sand with anterior and 

 posterior ends protruding. Beating cilia cause a stream of water to flow into the 

 mouth and out the gill slits, food particles being retained by entanglement with a 

 gelatinous secretion of the pharynx. 



