30 RESPIRATORY MECHANISMS 



in the economy of the Tubificidce. As far as can be judged from 

 his description, the amount of water entering the intestine 

 must be quite small, and when the O2 content is very low, as is 

 the rule, the amount of O2 to be obtained in this way is negli- 

 gible and it appears much more likely that respiration is al- 

 most exclusively cutaneous. It seems difficult to understand 

 how the worms avoid losing much oxygen by diffusion into the 

 mud from the fore part of the body, and it would be well 

 worth studying these worms by means of flagellate indicators 

 and perhaps the luminous bacteria, which will react to mini- 

 mum concentrations of 2 , to find out whether oxygen is 

 perhaps absorbed only through the tail while the fore part is 

 impermeable to the gas. A study of the affinity of Tubifex 

 haemoglobin for oxygen (cp. p. 104) would also probably yield 

 interesting results. 



In an echiuroid worm Urechis caupo living in burrows in 

 tidal flats along the Pacific coast of America and studied by 

 V. E. Hall (1931) and Redfield and Florkin (1931), ventila- 

 tion of the hind gut acting as a water lung has been definitely 

 established. The integument is very thick and ill adapted to 

 respiration. The hind gut is a large thin-walled sack, stretch- 

 ing along the whole length of the body. Water is taken into 

 this sack by a series of small inhalations and again expelled 

 by one large exhalation. Antiperistaltic waves run over the 

 hind gut all the time and mix both the water and the coelomic 

 fluid, which contains a large number of corpuscles carrying 

 haemoglobin. Hall measured on a 60 g animal a total ventila- 

 tion of the (artificial) burrow of 29 ml/minute when the animal 

 was feeding. When not feeding the worm would pump about 

 13 ml/m, of which one-half was taken into the hind gut where 

 about 1/3 of the oxygen would be utilized. 



Many of the fresh-water Pulmonata (Limn^a, Planorbis) can 

 live at considerable depths, and their lungs are then filled with 

 water. It is possible that they ventilate the lungs with water, 

 but much more likely that they obtain the oxygen through the 

 general surface. (Precht [1939] gives the references to 

 numerous, but not conclusive, studies.) Without mechanical 



