HABITS AND STRUCTURE OF ARENICOLA MARINA. 29 



on fig. 5, and PI. 3^ fig. 6. The whole middle region of the 

 body is devoid of septa, which, however, reappear on the last 

 two somites of the branchial region, and are present through- 

 out the tail in a complete form, though they are perforated to 

 allow of the more thorough circulation of the coelomic fluid. 



Arenicola fresh from the sand exhibits a series of peri- 

 staltic waves of the body-wall from behind forwards, which 

 can be easily seen if the gonads are sufficiently developed to 

 cause slight swellings, which each wave carries forwards. 

 These waves of fluid are probably of considerable physio- 

 logical value. They assist the circulation of the fluid, the 

 coelomic cells, and the developing reproductive cells. They 

 inflate the anterior digging part of the worm, and thus assist 

 in burrowing. By their action the contents of the gut will 

 tend to travel slowly backwards, the weak visceral muscu- 

 lature being probably insufficient by itself to cause the 

 requisite amount of movement of the sticky sand : while in 

 defsecation the main agent is doubtless the pressure of the 

 coelomic fluid on the intestine, brought about by violent con- 

 tractions of the body-wall. 



The coelom is lined by a very thin layer of flattened cells, 

 which undergo remarkable changes in certain parts of the 

 body, resulting in the formation of (1) chlorogogenous tissue, 

 (2) ova or spermatozoa, (3) coelomic corpuscles. 



The coelomic fluid is a mixture of sea-water and globulins, 

 among which only paraglobulin has hitherto been detected 

 (Krukenberg, 1882, p. 87). We find that the specific gravity 

 of the fresh fluid (including corpuscles) varies slightly, but is 

 on the average 1'0288.^ 



On exposure to air this fluid coagulates, and a delicate 

 fibrous network is formed, binding the corpuscles together. 

 If carmine is injected into the coelom, it is removed by the 

 coelomic corpuscles, by the cells lining the coelom and by the 



• It was found to be least (1-0270) in specimens which had been kept for 

 some time in sea water, and greatest (1-0311) in those whicii had been kept 

 for thirty-six hours in moist seaweed only. The specific gravity of the sea 

 water used was r0264. 



