Elements of zoölogy a Elements of zoölogy : a textbook elementsofzolo00tenn Year: c1875 Magnified portion of Pearl-shell. Magnified portion of Pinna. Magnified portion of Terebratula. not red. Their circulatory system consists of a heart, arteries and veins, and venous sinuses. Their respiratory organs are gills, or lung-like cavities. Their muscular system is well developed. Their nervous system is mainly FIG. 529. 'Nervous System in the Molhiaca. a, ft, c, d, nerve-ganglions. The principal center of the nervous system is in the form of a ring surrounding the gullut. in the form of a rin
Elements of zoölogy a Elements of zoölogy : a textbook elementsofzolo00tenn Year: c1875 Magnified portion of Pearl-shell. Magnified portion of Pinna. Magnified portion of Terebratula. not red. Their circulatory system consists of a heart, arteries and veins, and venous sinuses. Their respiratory organs are gills, or lung-like cavities. Their muscular system is well developed. Their nervous system is mainly FIG. 529. 'Nervous System in the Molhiaca. a, ft, c, d, nerve-ganglions. The principal center of the nervous system is in the form of a ring surrounding the gullut. in the form of a ring which surrounds the oesophagus or throat, from which nervous filaments radiate and con- nect with other nervous ganglia in other parts of the body. As to reproduction, most kinds of mollusks increase by means of eggs. Some kinds, as the River-Snails (Paludina), etc., are called viviparous, because they re- tain the eggs in the oviduct till after they are hatched, and the young have attained a considerable size. Some kinds of tunicate mollusks increase by budding.
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