322 UNITY AND DIVERSITY IN BIOCHEMISTRY 



not provided with a circulation like Urechis, the functioning of this 

 reserve can be clearly seen. The Echurian Urechis caupo is found along the 

 Californian coast living in the mud and sand in a U-shaped tube through 

 which it causes water to circulate during its periods of activity by the 

 peristaltic action of its musculo-cutaneous tube. The continually renewed 

 water brings food and oxygen to it. In fact by a peristaltic movement oppo- 

 site in direction to that of the musculo-cutaneous tube, the animal can 

 "inspire" the circulating water through its anus into its terminal intestine 

 which is highly developed and the thin wall of which is in contact with the 

 haemoglobin-containing corpuscles of the coelomic fluid. After a number 

 of these inspiratory movements, a single expiratory movement follows 

 expelling all the water in the intestine. In well-oxygenated water the haemo- 

 globin of the coelomic fluid is almost completely saturated (97%) although 

 the oxygen partial pressure is only 75mm Hg i.e. much less than in the 

 surrounding water (around 150mm). If the animal is at rest under these 

 conditions its haemoglobin will remain completely saturated and the 

 dissolved oxygen will be sufficient for its needs. As we have said, the animal 

 draws sea-water into its intestine and then expels it after having oxygen- 

 ated its coelomic fluid at the expense of the oxygen in the sea- water. (In the 

 expired water pQ_^ — 100mm approx.) Under these conditions it does not 

 use its haemoglobin at all. The volume of the coelomic fluid is about 20cm^ 

 and its oxygen capacity is 4 volumes per cent, so that the coelomic fluid 

 contains 0*8 cm^ of oxygen. The oxygen consumption of Urechis, on an 

 average, is around O-OlcmYmin. It is clear that only a small part of the 

 oxygen has to be replaced each minute and for this the dissolved oxygen 

 suffices. During its periods of activity which coincide with the periods of 

 elimination of water, Urechis lives on the dissolved oxygen provided by its 

 respiratory system. When the animal retires into the middle horizontal part 

 of its tube after a period of activity, it rem.ains motionless and ceases to 

 cause the water to circulate or to be taken into its digestive tube. 



The total amount of oxygen in the coelomic fluid and in the water of the 

 intestine corresponds to the amount which it consumed in 70 min; if the 

 coelomic fluid did not contain haemoglobin the oxygen would be used up 

 after 14 min. Thus the presence of haemoglobin enables the animal to have 

 a rest period between its periods of activity five times longer than would 

 otherwise be possible. 



When Urechis respires, the pQ^ of the water in its intestine is lOOmm 

 when the /)q^ of the outside water is 150mm. 



Under these conditions the pQ^ of the coelomic fluid is 75mm. These 

 conditions correspond to 97% saturation. The haemoglobin in the cor- 

 puscles remains saturated and the oxygen diffuses from the water in the 

 intestine to the blood plasma and from there to the tissues. There is no 

 evidence of the haemoglobin, which does not circulate, playing any part as 



