. Elementary text-book of zoology. Zoology. <)-} OBGANJZATIOX AND OF ANIMALS IX GENEBAL. ostia, provided with lip-like valves, which act so as to allow the blood only to enter the organ. From the heart, as central organ of the circulation, well defined canals, the blood vessels, are then developed, which in the Invertebrata may alternate with lacunae not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart Avhich are provided with independent walls, and developed into blood vessels (marine Copepoda, Calanella, fig. 53). A
. Elementary text-book of zoology. Zoology. <)-} OBGANJZATIOX AND OF ANIMALS IX GENEBAL. ostia, provided with lip-like valves, which act so as to allow the blood only to enter the organ. From the heart, as central organ of the circulation, well defined canals, the blood vessels, are then developed, which in the Invertebrata may alternate with lacunae not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart Avhich are provided with independent walls, and developed into blood vessels (marine Copepoda, Calanella, fig. 53). At a higher stage of development not only do these efferent vessels acquire a more complicated structure, but a part of the lacuna-system, especially in the neighbourhood of the heart, acquires a membranous invest- ment, and gives rise to vessels which carry the blood back to the ,P* FIG. 54.—Heart and blood vessels nnd gills of the crayfish. C, heart, in a blood sinus ; with Pa several pairs of ostia; Ac, cephalic aorta; Aab, abdominal aorta; An, sternal artery. pericardial sinus, from which it passes through the venous ostia into the heart (Scorpions, Decapods) (fig. 54). In other cases (Molluscs) the blood flows directly from the afferent vessels into the heart, the walls of the vessel being directly continuous with the walls of the heart. The heart in such cases consists of two chambers, the one known as auricle serves for the reception of the returning blood, the other known as ventricle for its propulsion (fig. 55). The vessels passing from the ventricle and carrying the blood from the heart are called arteries ; those returning the blood to it are called veins, and, in the higher animals, are distinguished from the arteries by their thinner walls. Between the ends of the arteries and the beginning of the veins the body cavity intervenes either as. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabi
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
