THE HABITS AND STRUCTURE OF ARENICOLA MARINA. Ill 



show nephridia from worms 29 - 5 and 44 mm. long, in which the 

 processes on the dorsal lip are being formed. In the post-larval stao-e 

 (Benham, 1893) the nephridia have no funnels, the development of 

 which has still to be investigated. 



10. Ccelom. 



The ccelom of Arenicola is well developed, and continuous in all its 

 parts. Not only does it form the space between the alimentary tract 

 and body-wall from one end of the body to the other, but it is carried 

 along with the blood-vessels into the intermuscular spaces. Thus the 

 blood-vessels of the prostomium, of the buccal sheath, and of the body- 

 wall generally are accompanied by coelomic canals which very probably 

 serve as lymphatic spaces from which nutritive matters can be absorbed 

 by the surrounding tissue, and into which waste nitrogenous substances 

 may be excreted. 



The segmentation of the body-cavity is very faintly marked. 

 Anteriorly three diaphragms, perforated just above the nerve- 

 cord, are present whose position and relations are indicated 

 on Fig. 5, and PI. VIII., 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 throughout 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 peristaltic 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 con- 

 siderable physiological value. They assist the circulation of the fluid, 

 the ccelomic 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 musculature being probably insufficient 

 by itself to cause the requisite amount of movement of the sticky sand : 

 while in defalcation the main agent is doubtless the pressure of the 

 coelomic fluid on the intestine, brought about by violent contractions of 

 the body-wall. 



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



