x PHYLUM ANNULATA 495 



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 each side several pairs 

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

 digitate processes lying in the ccelome. The cesophageal connectives 

 (coins, ce) 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 

 (I., II.) ; on section it appears distinctly double. Two delicate 

 muscular bands (Fig. 388, in. 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 folded and pigmented walls, 

 which opens in the middle line of the dorsal surface just behind 

 the tentacular fold (Fig. 387, cer. org.), extends backwards 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 entirely absent. The digi- 

 tate 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. 388, neph.) are situated tolerably far forwards, the external 

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

 nearly straight tubes, of a brown or yellowish colour, and very 

 mobile in the living condition. Near the external opening, which 

 is situated at the anterior end, is the internal opening into the 

 coelome. The sexes are separate. There are no definite gonads 

 except at a certain season of the year, when cellular elevations 

 developed in the connective tissue covering the ventral 

 retractor muscles of the introvert represent ovaries or testes 

 as the case may be. These give origin to cells which become 

 detached and develop into the fully-formed sexual elements 

 while floating about in the coelomic fluid. The nephridia act 

 as gonoducts. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Gephyrea are Annulata with the body devoid of any ap- 

 pearance of segmentation in the adult condition. There is a large 

 coelome, which is not divided into chambers by mesenteries or 

 septa. A blood- vascular system is sometimes present, sometimes 

 absent. The ventral nerve-cord is not composed of a chain of 

 ganglia. There is usually only one pair of nephridia. The sexes 



