454 ZOOLOGY SECT. 



Anteriorly and dorsally it gives off a number of digitate pro- 

 cesses lying in the coelome. The cesophageal connectives (corns, ce) 

 which it gives off behind are greatly elongated ; each gives off 

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

 ing 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. 354 m. n. co.) } which take orgin 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. 353 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 which like those 

 of the majority of the Polychceta are of the character of diplo- 

 nephridia. These (Fig. 354, ncpli.) 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 cover- 

 ing 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 ccelomic fluid. The segmental organs 

 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 

 ccelome, 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 scx< > 

 are separate, the gonads simple, and the nephridia act as 

 gonoducts. 



