96 



THE BIOLOGY OF MARINE ANIMALS 



body cavity rise to 36 cm during muscular contraction, and to 27 cm during 

 burrowing. Removal of a quarter of the coelomic fluid more than doubles 

 burrowing time. Higher pressures are developed in the anterior branchial 

 region than in posterior segments. This results from contraction of circular 

 muscle in the posterior branchial region, driving fluid forwards in the 

 anterior region, which is then sealed off by constriction of the body wall 

 against the gut. In the ragworm Neanthes pressure of coelomic fluid rises 

 from a resting value of 1-2 cm to 15 cm H 2 during activity; fluid pres- 

 sures in Glycera are 0-5-2 cm. at rest, and 8 cm H 2 when the animal is 

 active (Chapman and Newell (24), Zuckerkandl in Prosser (108)). 



In sipunculoids high internal pressures are attained which provide body 

 turgor essential for burrowing. In Sipunculus nudus and Golfingia 



So 



13 8 



41 



7 4 8 



4 3 cm H 2 



1 t 1 t t tt ! f t t I 



3 4 



67 



8 9 10 



11 12 



8 12 16 20 



Time (minutes) 



24- 



Fig. 3.3. Hydrostatic Pressure and Movement in My a arenaria 



Record showing movement of shell valves and simultaneous readings of internal 

 hydrostatic pressure of mantle fluid. Movement upwards indicates adduction. Elonga- 

 tion of the siphons was noted at points 1-12. Each peak on the trace was accompanied 

 by an elongation of the siphons until they reached a length of 40 cm. (Redrawn from 

 Chapman and Newell (25).) 



gouldii fluid pressures increase from 2-3 cm H 2 in the relaxed condition 

 to around 100 cm during maximal activity. When lying freely in the water 

 and executing burrowing movements coelomic pressure in Golfingia 

 rises to 25 cm and in Sipunculus to 40 cm H 2 0. Pressures are higher during 

 fast burrowing in sand, around 90 cm H 2 in Sipunculus. Burrowing is 

 accomplished in these animals by protracting the proboscis and driving 

 it into the sand, the loosened sand being carried backwards behind the 

 animal. During burrowing activity there are regular rises and falls of pres- 

 sure in the coelomic fluid, accompanied by eversion and retraction of the 

 proboscis. Protrusion of the proboscis usually occurs during a rise of 

 coelomic pressure; the proboscis can be withheld or retracted against high 

 coelomic pressures by retractor muscles, however. When the body wall is 

 perforated, fluid pressures can no longer be built up and eversion of the 

 proboscis ceases (108, 142). 



