x PHYLUM ANNULATA 487 



which perhaps acts, like haemoglobin in many animals, as a carrier 

 of oxygen ; (3) reproductive elements ; (4) peculiar unicellular 

 ciliated bodies, the urns, which are developed by proliferation from 

 cells on the wall of the dorsal blood-vessel. These are comparable 

 in structure and function with the ciliated funnels of the Hirudinea 

 (q.v.), 



The blood-vascular system is very feebly developed. It 

 consists of dorsal and ventral contractile vessels, the former known 

 as the ' heart," communicating in front with a circular sinus 

 at the base of the tentacular fold. 



The alimentary canal (Fig. 403) is a cylindrical tube of 

 uniform character throughout. It is twice the length of the body, 

 running back from the mouth towards the posterior end, and then 

 bending sharply round to run forwards to the anus, the two limbs 

 being twisted spirally round one another. Running along the inner 

 surface of the entire length of the alimentary canal, with the excep- 

 tion of the terminal part or rectum, is a narrow groove. Connected 

 with the rectum is a narrow ccecum of variable length, which opens 

 into the beginning of the rectum. Two tuft-like groups of rectal 

 glands occur close to the anal opening. 



The nervous system (Fig. 404) differs considerably from that 

 of the rest of the Annulata. There is a relatively small bilobed 

 cerebral ganglion situated on the dorsal aspect just behind the 

 tentacular circlet, to which it gives off on ea.ch side several pairs 

 of nerves. Arising from it anteriorly and dorsally are a number of 

 digitate processes lying in the ccelome. The orsophageal connectives 

 (corns. 03} which it gives off behind are greatly elongated ; from each 

 arise muscular nerves (n. mu. ret), and also a visceral nerve (n. spl) 

 passing to the alimentary canal. The two commissures unite behind 

 to form a ventral cord, which extends throughout the rest of the 

 length of the body. The ventral cord presents no appearance of 

 ganglia : it sends off laterally a large number of pairs of nerves 

 (/., 77.) ; on section it appears distinctly double. Two delicate 

 muscular bands (Fig. 403, m. n. co.), which take origin anteriorly 

 from the body-wall, become attached to the nerve-cord, and follow 

 it throughout its length, giving off small branch-bands to accom- 

 pany the lateral nerves. A canal with ciliated, folded and pig- 

 mented walls, which opens in the middle line of the dorsal surface 

 just behind the tentacular fold (Fig. 402, cer. org.), extends back- 

 wards to the anterior ventral surface of the cerebral ganglion, where 

 it ends blindly. It is possible that this, the cerebral organ, may be 

 a sensory organ of some kind. Eyes are wanting. The digitate 

 processes of the cerebral ganglion, which bear a number of ciliated 

 cups along their edges, may be sensory in character. 



Sipunculus has only a single pair of nephridia. These 

 (Fig. 403, 'ncplt) are situated tolerably far forwards, the external 

 openings being about 2 cm. in front of the anus. They are long, 



