466 F. W. GAMBLE AND J. H. ASHWORTH. 



is usually surrounded by a vacuole " (Picton, 1898, p. 

 289). 



This quotation is applicable word for word to tlie structure 

 of the lieart-body of Arenicola Grubii, so that in this 

 respect and also in its mode of origin the heart-body, 

 although absent in A. cristata and A. Claparedii, shows a 

 close agreement witli certain Terebellids (Lanice conch i- 

 lega, Polymnia nebulosa). Only in its point of origin 

 does it diifer conspicuously from all other Polychgetes, in 

 which it is always unpaired and usually in the dorsal vessel/ 

 whilst in Arenicola it is formed in each of the paired 

 contractile " hearts " which unite the gastric plexus or sinus 

 and the ventral vessel. There is, of course, no a priori 

 reason why an involution, followed by proliferation and 

 resulting in a "heart-body," should not occur on any of the 

 larger vessels, unless its occurrence is determined by some 

 mechanical function which it may subserve. Now in 

 Ampharetidae (Fauvel [1897] ), Terebellids, and Cirratulidae, 

 a short dorsal vessel arises from the gastric sinus, and ends 

 by giving off pairs of branchial vessels, and both Picton and 

 Fauvel have concluded by direct observation that the heart- 

 body ensures the whole of the blood at each systole passing 

 on to the gills. In Arenicola, however, its function cannot 

 be thus explained. There is no cardiac body in the dorsal 

 or ventral vessel in the branchial region or on any part of 

 their course, but only on the vessel connecting on each side 

 the gastric plexus and the ventral vessel. The heart-body, 

 in fact, appears to be a means of preventing regurgitation of 

 the blood into the gastric plexus after systole, and of ensuring 

 its passage into the ventral vessel. Whether, in addition, it 

 performs important excretory functions can only be decided 

 when more data shall have been collected on the nature of 

 the substances collectively termed chloragogen. 



' In Pectinaria Clapaiede and Wiren have described a cardiac body in 

 the ventral vessel. 



