The blood circulation of animals possessing chlorocruorin

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Abstract
Most blood vessels of sabellids and serpuiids are rhythmically contractile, including all the blind-tending capillary vessels in crown, body wall, and celom. In the continuous vessels the blood is circulated by peristalsis, in the capillaries there is an ebb and flow of the blood. The rhythmic contractions of the vessels are independent of the central nervous system. The simultaneity of contractions in the vessels of the separate filaments of the crown is controlled by a ganglion at the base of each half of the crown. After sabellid worms have retired into their tubes for over half an hr., the contractions of their blood vessels cease. The contractions recommence as soon as even the tip of the crown projects beyond the end of the tube. In spite of the cessation of blood circulation, Spirographis can remain uninjured for 8 hrs. in its tube when the latter is artificially closed. After 8 hrs. the animal makes a new anterior lateral opening in the closed tube and re-expands its crown. When sabellids or serpuiids, removed from their tubes, are put into sea water the pH of which has been brought below 6.0 by the addition of CO2, the contractions of their blood vessels stop. Neither water acidified with HC1 to the same pH as the water containing CO2, nor water lacking dissolved O2, produces this effect. The inhibition of blood vessel contractions due to CO2 is reversible when the animals are replaced in pure sea water. CO2 does not inhibit the blood vessel contractions through a nervous reflex. It causes an internal accumulation of CO2 which acts directly on the blood vessel walls. It is probable that the cessation of blood vessel contractions when sabellids have retired into their tubes is due to the accumulation of CO2 in the water between the filaments of the crown. The threshold pH for the inhibition of blood vessel contractions of Dasychone was determined in (a) sea water containing dissolved CO2, (b) sea water of abnormally high alkali reserve containing CO2, and (c) sea water acidified with HC1. From these data it was concluded that both CO2 and hydrogen ions are concerned in the inhibition, but that the former is more important than the latter. CO2 also causes the reversible inhibition of contractions in the following cases: (a) the dorsal blood vessel of Nereis; (b) the hearts of Daphnia, Artemia, Chloeon nymphs, Phallusia, and chick embryos; (c) the contractile vesicles of Limax embryos.

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