. A text-book in general physiology and anatomy. Physiology, Comparative; Anatomy. XVII. SKELETONS OF THE LOWER ANIMALS.^ The most striking point of difference between the higher and the lower animals is the difference in their skeleton or supporting framework. In fact, it is customary to sub- divide all animals into two groups (vertebrates and inver- tebrates) on the basis of the presence or absence of a backbone. Invertebrate animals are characterized either by the entire absence of skeletal structures, as in the slugs, worms, jellyfishes, etc., or by the presence of an external protec- tive
. A text-book in general physiology and anatomy. Physiology, Comparative; Anatomy. XVII. SKELETONS OF THE LOWER ANIMALS.^ The most striking point of difference between the higher and the lower animals is the difference in their skeleton or supporting framework. In fact, it is customary to sub- divide all animals into two groups (vertebrates and inver- tebrates) on the basis of the presence or absence of a backbone. Invertebrate animals are characterized either by the entire absence of skeletal structures, as in the slugs, worms, jellyfishes, etc., or by the presence of an external protec- tive layer called an external skeleton. A few, such as. Fig. 101 — Protozoans (foraminifera) with external skeleton. the cuttle fishes, have an internal framework, but it is not in anjr way similar to the internal skeleton of the vertebrate animals. Vertebrates all possess an internal ' See Footnote, p. 146, Chapter X. 252. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Eddy, Walter Hollis, b. 1877. New York, Cincinnati [etc. ] American book Co
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