RESPIRATION 145 



Respiration in Hind-gut and Diverticula. Respiratory mechanisms 

 exist in some species for pumping water into and out of the hind-gut. In an 

 echiuroid worm, Urechis caupo, the long hind-gut is employed as a respira- 

 tory organ. Water is pumped into the hind-gut and out again by contrac- 

 tions of the muscular cloaca. The thin walls of the hind-gut function as a 

 respiratory surface, and gases are exchanged with the coelomic fluid, which 

 is kept in motion by anti-peristaltic waves passing along the gut wall. 

 A non-feeding animal takes into the hind-gut about half the water pumped 

 through its burrow (75). 



Holothurians also possess water-lungs. These are respiratory trees, 

 which form a system of highly ramified tubules originating in the cloaca 

 and extending into the coelom. Water is pumped alternately in and out 

 of this system by rhythmic contractions of the body wall, and gases are 

 exchanged with coelomic and vascular fluids. In Holothuria grisea about 

 10 c.c. of sea water is driven into the cloaca-branchial tree system every 

 two minutes, followed by expulsion (Fig. 3.4). The respiratory tree of 

 H. tubulosa is responsible for 50-60% of the respiratory exchange of the 

 body. When the water in which it is living becomes stagnant, Holothuria 

 raises its posterior end to the surface, and takes air into the cloaca, a 

 behaviour pattern similar to that of Arenicola under comparable condi- 

 tions of oxygen stress (18, 19, 121). 



Water Circulation in Tubes and Burrows. Animals which burrow 

 or live in tubes or galleries must have access to the outside sea water for 

 respiration. Some species, such as burrowing lamellibranchs, extend their 

 siphons to the surface. Cryptocephalous polychaetes protrude cephalic 

 processes. Many other animals possess some means of circulating water 

 through their tenements for respiratory, and often for feeding, purposes. 



A burrowing polychaete with a peculiar respiratory mechanism associ- 

 ated with subterranean habits is the sea mouse Aphrodite. This animal 

 burrows in muddy sand, while retaining its posterior end at the surface, 

 and in this position maintains a ventilation current of sea water over its 

 body (Fig. 4.5). The dorsal surface bears a series of overlapping scales or 

 elytra, which are covered in turn by a dense feltwork of bristles. By eleva- 

 tion of the ventral surface, water is drawn along the underside of the body 

 from the exposed tail. During inspiration the ventral surface is depressed, 

 the elytra raised, and water streams upwards between the parapodia into a 

 respiratory space lying underneath the elytra. Respiratory exchange takes 

 place over the whole exposed integument. Water is expelled by depressing 

 the scales and driving it out posteriorly over the upper surface of the tail (31). 



The heart urchin Echinocardium digs burrows some 15-20 cm deep, and 

 maintains a channel open to the surface (Fig. 4.6). Respiratory currents 

 are created by cilia, and tube feet of the aboral surface are enlarged as 

 gills (28). 



Peculiarities of respiratory mechanisms associated with burrowing 

 habits are recorded for many crustaceans. The gribble Limnoria, which is 

 a wood-borer, keeps its gallery in communication with the outside sea 



