GEPHYREA 167 
The vascular system is more complex in the Chaetifera 
than in the Achaeta. The dorsal vessel in Hehiwrus opens 
behind into a circular blood-vessel which surrounds the oeso- 
phagus. At its anterior end it enters the prostomium and runs 
to the tip of this organ, here it splits, and the two branches 
return, one down each side of the prostomium, till they have 
passed the mouth, when they unite to form a median supra- 
neural blood-vessel. This is connected with the peri- 
oesophageal circular ring by a transverse vessel. Haemo- 
globin has been detected in the coelomic corpuscles of 
Thalassema., 
The last-named genus may have from one to four pairs of 
nephridia, according to the species, Zchiwrus has usually two 
pairs. 
The nervous system, like the vascular system, is continued 
into the prostomium, running all round the edge, and finally 
uniting below the oesophagus, thus forming a circum-oeso- 
phageal ring, which gives off the ventral cord. In no place is 
the nerve ring or cord thickened to form anything like a 
ganglion. 
In #ehivrus and the female Bonellia the coloemic epi- 
thelial cells which surround the ventral vessel enlarge and 
form the reproductive cells, which are thus favourably situated 
for receiving nourishment. 
There is a very remarkable dimorphism in the genus 
Bonellia. The female is a fair-sized animal, with a body 2 
inches long, but the male is a microscopic planarian-lke 
organism which lives in a recess of the nephridium of the 
female. It is from 1 to 5 mm. long, and is ciliated all over. 
Its intestine is not functional, and it ends blindly both in front 
and behind. The spermatozoa arise from the coelomic epi- 
thelium, and escape by a modified nephridium. A nervous 
system; but no vascular system, is present. 
The male larva is said to cling to the prostomium of ‘the 
female, and thence to pass into the mouth, where it undergoes 
its final changes, then it creeps out from the mouth and into 
the nephridium, where it spends the rest of its life. Another 
genus, Hamingia, has a similarly degenerate male, which also 
lives in the nephridia of the female. 
