BODY FLUIDS AND CIRCULATION 



95 



burrowing, the haemolymph supplies the necessary turgor for movements 

 to be executed. When the foot is extended the pedal muscles relax and 

 blood flows through the pedal artery into the foot. Retraction of the foot 

 is brought about by contraction of the pedal muscles, and blood is shifted 

 largely into spaces in the mantle. Burrowing movements are carried out 

 with great rapidity by the razor-shell Ensis. In downward burrowing the 

 foot is extended into the sand, blood flows into it and the tip swells out 

 into a bulbous disc (Fig. 3.2). This acts as an anchor while the pedal 

 muscles contract and draw the animal down. 

 This is followed by return of the blood to the 

 body, while the foot extends once more, re- 

 peating the manoeuvre. In upward progression 

 the pedal muscles remain relaxed while the 

 tip of the foot becomes distended with blood. 

 With the tip anchored, blood is forced into 

 the upper region of the foot, which elongates, 

 thus pushing the animal upwards. The con- 

 secutive muscular contractions involved in 

 burrowing can still take place in an ex- 

 sanguinated animal, but in the absence of 

 turgor these are weak and ineffective (42). 



Mechanisms used in siphonal extrusion vary 

 in different species of lamellibranchs. The 

 siphons of Mya are extended by water forced 

 into their lumina from the mantle cavity. The 

 mantle and siphonal cavities form a fluid-tight 

 system, and force is applied by contractions 

 of the adductor muscles. Elongation proceeds 

 stepwise; water is taken into the system be- 

 tween successive elongations, so that the in- 

 ternal volume remains constant (Fig. 3.3). In 

 Scrobicularia, in contrast, siphonal extension 

 is effected by the intrinsic musculature of the 

 siphonal walls acting on the blood within, 

 the volume of which remains constant (25). 



In tubular animals, such as annelids, echiuroids, sipunculoids, holo- 

 thurians and enteropneusts, pressure of the body fluids is determined by 

 tonus and contraction of the body wall. In these soft-bodied animals the 

 body fluids constitute a hydrostatic skeleton against which the muscle 

 systems can operate. In burrowing species, activity and burrowing move- 

 ments are accompanied by increased turgor and elevation of fluid pressures 

 in coelomic cavities. Muscle and fluid systems form a co-ordinated unit, 

 proper changes in each of which are necessary for locomotion and burrow- 

 ing to be accomplished. 



In the burrowing lugworm Arenicola, pressure in the coelomic cavity 

 when the animal is at rest averages 13-6 cm H 2 0; pressures in the anterior 



'»u»HH)nm)»inn»nmwi)n 



Fig. 3.2 Razor-shell 

 Ensis. Sequence of Move- 

 ments in Downward 

 Burrowing. (Redrawn 

 from Fraenkel (42).) 



