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. Sipunculus 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 oblique muscles les 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 
