ADELOCHORDA 



191 



Neural Cavity. — The upper or neural cavity contains the brain 

 and the spinal cord. 



Hemal Cavity. — Below the vertebral column with its neural 

 cavity is the large cavity of the body, the hemal cavity, which 

 contains the heart, lungs, digestive organs, and other viscera. 



Skeleton. — Most of these higher forms have an internal bony 

 skeleton or a cartilaginous one, as in some fishes. The vertebral 

 column, or backbone, is composed of a varying number of bones, 

 each called a vertebra, hence the branch is named Vertebrata, 

 or, if named from the notochord, Chordata. 



Divisions of the branch are usually 

 made to distinguish the primitive groups 

 (Fig. 152) or Protovertehrates, from the 

 true Vertebrates. 



The Protovertehrates consist of three 

 separate groups or sub-phyla, not closely 

 related to each other, but each, in a 

 primitive way, is entitled to relationship 

 with the Chordata or Vertebrata. 



SUB-PHYLUM AND CLASS I. 

 ADELOCHORDA 



The Balanoglossus is the principal 

 genus of this group, though two deep- 

 sea forms (Rhabdopleu'ra and Ceph'alo- 

 dis'cus) have a notochord, and the latter 

 has a pair of gill-slits, but in other ways 

 they are like the polyzoans. The Bal- 

 anoglossus (Fig. 154) is a small marine 

 chordate. Its surface is ciliated. It is 

 from 1 to 4 or 5 inches in length, and, by 

 means of its proboscis, burrows in the 



mud along the seashore. A study of the animal or of a good 

 figure will show that it has (1) a dorsal nerve cord, (2) a 

 notochord, and (3) gill-slits. 



Body Regions. — The Balanoglossus is divided into three 

 body regions: the proboscis, a club-shaped hollow anterior por- 

 tion opening exteriorly by a single pore; back of the proboscis 



Fig. 154. — Balano- 

 glossus: p, Proboscis; 

 c, " collar "; gs, gill- 

 slits; enlarged. (From 

 Dodge's " General Zo- 

 ology," American Book 

 Co., Publishers.) 



