GEPHYR^A 167 



The vascular system is more complex in the Chaetifera 

 than in the Achaeta. The dorsal vessel in Echiurus 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, tiU 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 

 TJialassema. 



The last-named genus may have from one to four pairs of 

 nephridia, according to the species, Echiurus 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 riag or cord thickened to form anything like a 

 ganglion. 



In Echiurus 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-like 

 organism which Hves 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 bKndly 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 chng 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. 



