ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 467 



10. Coelom. 



Thecceloraof A. marina and A., cristata is very spacious, 

 that of A. Clap are clii moderately so, but in A. Grubii and 

 A. ecaudata it is comparatively small. As in A. marina 

 the middle region of the body is devoid of septa, but, as 

 noticed above, there are structures in A. Grubii which may 

 be regarded as rudimentary septa, viz. the thin sheet of con- 

 nective tissue which supports the afferent nephridial vessel 

 on its course from the ventral vessel to the nephridium (PI. 

 26, figs. 53, 54). In all species there are in the posterior 

 region of the animal septa only incomplete ventrally. 



In the fresh condition A. Grubii and A. ecaudata show 

 the peristaltic waves which pass along the body from behind 

 forwards even better than A. marina. As pointed out in 

 our account of A. marina (1898) this peristaltic motion is of 

 considerable importance in promoting the efficient circulation 

 of fluid in the coelom; in inflating the anterior portion, thus 

 aiding in burrowing ; in assisting the gut muscles to cause 

 the backward motion of the sand in the alimentary canal ; 

 and in assisting defeecation. 



The coelomic fluid of all species resembles that of A. 

 marina. It contains coelomic cells and reproductive pro- 

 ducts. Coelomic cells are tolerably abundant in all species, and 

 consist of fusiform cells about 40 ju in length, and smaller 

 more spherical or amoeboid cells. On the coagulation of the 

 ccBlomic fluid, which occurs very soon after its exposure to 

 the air, the fusiform cells and to a less extent the amoeboid 

 cells unite with the fibrous network which is formed during 

 coagulation to form a clot. Fusiform cells are fewest in our 

 specimens of A. cristata, and most abundant in A. Grubii. 



Instead of floating freely in the coelom, a great proportion 

 of the gonads present in the coelomic fluid accumulate in 

 the space between the oblique muscles and the body-wall. 

 In A. Grubii and A. ecaudata they accumulate in the 

 posterior region of the body, well behind the nephridia. 



'I'he coelom is lined throughout by a layer of flattened 



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