. Principles of modern biology. Biology. 370 - Multicellular Animals, Especially Man INF. VENA CAVA. TUNIC CORTEX PYRAMID PELVIC CHAMBER MEDULLA RENAL VEIN RENAL ARTERY Fig. 20-1. Diagram of the human urinary system, posterior view. becomes less and less adequate as the mass of an animal increases. In flatworms, such as Planaria, the body encompasses a consider- able mass of mesodermal tissues, intervening between the ectoderm and endoderm. Such an increase in the mass of the body requires the development of specialized excretory organs, and planarians possess a large num- ber of flame cells (
. Principles of modern biology. Biology. 370 - Multicellular Animals, Especially Man INF. VENA CAVA. TUNIC CORTEX PYRAMID PELVIC CHAMBER MEDULLA RENAL VEIN RENAL ARTERY Fig. 20-1. Diagram of the human urinary system, posterior view. becomes less and less adequate as the mass of an animal increases. In flatworms, such as Planaria, the body encompasses a consider- able mass of mesodermal tissues, intervening between the ectoderm and endoderm. Such an increase in the mass of the body requires the development of specialized excretory organs, and planarians possess a large num- ber of flame cells (Fig. 20-2), scattered throughout the mesoderm. Each flame cell is a hollow structure with a tuft of active cilia in the central cavity; and the beating of these accounts for the "flickering" that can be seen in a flame cell when it is viewed with a micro- scope. The many flame cells on each side of the planarian are connected by a branching system of fine tubes to main excretory ducts. The excretory ducts run anteriorly on either side of the mid-line of the body, emptying through a series of excretory pores, on the dorsal surface of the animal. Precisely how the flame cells function is not well understood. Apparently these cells extract water and other excretory wastes from the body fluid—which fills the tissue TUFT OF CILIA. 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 Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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