. Principles of economic zoo?logy. Zoology, Economic. 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
. Principles of economic zoo?logy. Zoology, Economic. 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 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.). Please note that these images are extracted from scanned page images that may have been di
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