. The biology of marine animals. Marine animals; Physiology, Comparative. 106 THE BIOLOGY OF MARINE ANIMALS ing a blood volume of 75 and a cardiac rate of 100 beats per min can turn over its entire blood volume in 3-8 min. The heart is the main pump- ing mechanism in decapod crustaceans, but body movements assist to some extent in propelling blood through the gills and back to the heart (20). In solitary ascidians such as Ciona the heart is a simple /^-shaped tube. The walls are composed of curious muscle fibres, differentiated into an. Fig. Myogram of the Heart of Palinurus, Stimu
. The biology of marine animals. Marine animals; Physiology, Comparative. 106 THE BIOLOGY OF MARINE ANIMALS ing a blood volume of 75 and a cardiac rate of 100 beats per min can turn over its entire blood volume in 3-8 min. The heart is the main pump- ing mechanism in decapod crustaceans, but body movements assist to some extent in propelling blood through the gills and back to the heart (20). In solitary ascidians such as Ciona the heart is a simple /^-shaped tube. The walls are composed of curious muscle fibres, differentiated into an. Fig. Myogram of the Heart of Palinurus, Stimu ed with Induction Shocks at Various Stages of the Cardi Cycle Stimulus ineffective at the beginning of systole (a), but effective during diastole (b, c, d). (From Carlson, 1906.) inner striated, and an outer sarcoplasmic portion. There is no endothelium. Periodically, the heart reverses the direction of its beat, at the same time reversing the course of circulation about the body. Alternation of direction of heart beat is characteristic of all tunicates—salps, pyrosomae and ascidians. Blood pressure is very low, around 2 mm Hg in Ascidia (36, 90). Ampullar Hearts Accessory devices for propelling blood through peripheral channels sometimes take the form of contractile ampullae. In cephalopods the branchial hearts are booster devices which drive systemic venous blood through the gills towards the systemic heart. The walls consist of spongy tissue lined with faintly striated endothelial cells; exit and entrance are guarded by sets of valves. The two branchial hearts contract simultane- ously and rhythmically (121, 127). In lancelets there a re small contractile bulbils at the bases of the gill bars. Lymph vessels of fishes sometimes bear contractile lymph hearts which drive lymph into the veins. In the tail of Angui/Ia, for example, there is a lymph heart which opens into the caudal vein. Lymph hearts are composed of striped anastomosing muscle fibres, and are provided with valv
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