Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 02 OIIOAKJZATION AKD DEVELOPMENT OF ANIMALS IN GENERAL, 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 thendeveloped, which in theInvertebrata may alternate with lacunse not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart which ar
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 02 OIIOAKJZATION AKD DEVELOPMENT OF ANIMALS IN GENERAL, 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 thendeveloped, which in theInvertebrata may alternate with lacunse not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart which 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- UKiit, and gives rise to vessels which carry the blood back to the Fig. and blood vessels and gills of the crayfish. C, heart, in a blood sinus ; with Ps several pairs of ostia; Ac, cephalic aorta; , abdominal aorta; Ag, 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 cidled 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
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