GEPHYREA 165 



each. Laterally the two lobes are continued into stout 

 nerves which embrace the oesophagus, and fuse to form a 

 ventral cord. On each side the ventral cord is supported by 

 two longitudinal muscles, and the whole is loosely attached to 

 the ventral surface of the body-wall by muscular strands. 

 The cord shows but slight traces of double origin, it bears no 

 ganglia, but ganglion cells are uniformly distributed on its 

 ventral surface. It gives off a series of lateral nerves, which 

 form complete rings round the body, situated in the skin 

 (Figs. 103 and 105). 



Two pits of large ectodermal cells, crowded with dense 

 black pigment, have sunk on each side into the brain. They 

 are hollow, and contain a coagulum in dead specimens. They 

 are usually spoken of as eyes. 



Phymosoma is dioecious. Both the ovary and testis are 

 formed of a ridge of the peritoneal epithelium which runs 

 across the body at the base of the ventral retractor muscles. 

 Certain of the cells of this ridge break off and float in the 

 coelomic fluid. In the female they become ova, in the male 

 they are the mother cells of the spermatozoa. The ova grow 

 a good deal whilst in the body-cavity, and secrete a thick egg 

 shell ; ultimately they leave the body through the nephridia. 

 The spermatozoa derived from one mother cell always remain 

 connected as long as they are in the body-cavity, and in this 

 condition are taken up by the funnel-shaped internal openings 

 of the nephridia. The ova are fertilised externally in the 

 water. 



Certain of the Gephyrea achaeta differ in many points 

 from Phymosoma. Sipvnmdus has no lophophore, and the 

 mouth is surrounded by a frayed fringe, which, like the 

 tentacles of other forms, is well supplied with nerves and 

 blood-vessels. 



Many species are without the hooks on the introvert. 

 A layer of obhque muscles lies very commonly be- 

 tween the circular and longitudinal fibres. The capacity 

 of the dorsal vessel, which acts as a reservoir for receiving 

 the blood when the tentacles and head are retracted, is in- 

 creased in some species of Phymosoma by a number of lateral 

 diverticula, and in some Sipunculids by the addition of a 



