262 J. H. ASHWORTH. 



and their structure is difficult to determine. The walls are 

 composed of a layer of peritoneal epithelium within which a 

 very thin sheet of muscle-fibres may be distinguished. In 

 some sections an exceedingly delicate endothelium appears 

 to be present, but this is difficult to distinguish with cer- 

 tainty. There is no trace of heart body such as is present in 

 the dorsal vessel of some other Polychaetes. 



Danielssen states that the blood-reservoir and the heart 

 are contractile, alternately expanding and contracting with 

 considerable force, driving the blood forward to the gills. 

 The blood-plasma is red and the corpuscles are few in 

 numbers. They are spherical or ellipsoidal cells, 6 — 9 /i in 

 diameter, and have prominent nuclei. It is very difficult to 

 ascertain where they are formed, but apparently some arise 

 from the cells lining the wall of the dorsal vessel, especially 

 in the region of the heart and blood-reservoir. In one speci- 

 men there is a mass of coi'puscles in the ventral vessel imme- 

 diately behind the fourth diaphragm. These corpuscles re- 

 semble in appearance and in reaction to stains the cells lining 

 the wall of the vessel in their immediate neighbourhood. 

 Possibly corpuscles are formed at various points in the vessels. 



13. Gills. 



The four pairs of gills are shrubby, much-branched out- 

 growths of the body-wall situated immediately above and 

 behind the notopodia of the second, third, fourth, and fifth 

 chsetigerous segments (fig. 1). 



Kathke describes the gills as being present on the fourth 

 to seventh segments, and figures three chajtigerous segments 

 anterior to the first pair of gills ; but other authors describe 

 the gills as being situated on the second to fifth segments. 

 It seems unlikely that Rathke's specimen, while agreeing 

 very closel}^ with other specimens obtained from the same 

 locality, should differ from these only in the position of the 

 gills. It seems. probable that in this respect Rathke's ac- 

 count is incorrect. 



The first gill is considerably smaller than the other three. 



