. The animals and man; an elementary textbook of zoology and human physiology. Zoology; Physiology. THE VERTEBRATES 193 The remaining three classes include a number of strange marine forms which until recent years were considered as worms, but which are now known to be the nearest living allies of the earliest or primitive vertebrates. The rela- tionship of these forms to vertebrates is manifest, not in the appearance or structure of the adult stage, but only during embryonic or larval stages. The ascidians.—The sea-squirts, or Ascidi- ans, common on the seashore, compose one class of these pr
. The animals and man; an elementary textbook of zoology and human physiology. Zoology; Physiology. THE VERTEBRATES 193 The remaining three classes include a number of strange marine forms which until recent years were considered as worms, but which are now known to be the nearest living allies of the earliest or primitive vertebrates. The rela- tionship of these forms to vertebrates is manifest, not in the appearance or structure of the adult stage, but only during embryonic or larval stages. The ascidians.—The sea-squirts, or Ascidi- ans, common on the seashore, compose one class of these primi- tive chordate animals. They possess a sim- ple, sac-like body (fig. 95), fastened to the rocks by one end, the other being provided with two openings, one for the ingress and the other for the exit of water, a strong current of which flows constantly through the body. By means of this current the ascid- ian obtains food. Usually sea-squirts live together in large colonies, and in some cases a number of individuals enclose themselves in a common gelatinous mass, forming what is called a compound ascidian. The ascidian when born is a tiny, free-swimming, tadpole- like creature with a slender finned tail. It swims about freely for only a few hours, however, soon attaching itself to a rock, and jn it§ further development becoming degenerate,. Fig. 9S. An ascidian, or sea-squirt, from the coast of California. (Natural size; after Jordan and Kellogg.). 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 Kellogg, Vernon L. (Vernon Lyman), 1867-1937; McCracken, Mary Isabel. New York, H. Holt and Company
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