175 
situated on the dorsal aspect of this part of the brain are 
in close association with the epithelium of the middle part 
of the prostomium. 
The posterior lobes are small and tapering. On 
tracing them backwards, the cells are seen to decrease in 
-quantity, and each lobe is continued as a fibrous tract, 
accompanied by a thin covering of cells, which hes on the 
inner side of the nuchal organ just below its sensory 
epithelium (figs. 84, 47). The posterior lobes are separated 
by a ccelomic space, containing muscles and blood-vessels., 
The brain has a strong ventral neurilemma sheath, 
especially on the anterior lobes, into which some of the 
prostomial muscles are inserted. Other muscles pass 
between the anterior lobes, and are inserted into the con- 
nective tissue underlying the epidermis. The brain 
derives its blood supply from the dorsal vessel, small 
branches of which break up into capillaries on its ventral 
surface. 
On examining a number of specimens of gradually 
increasing sizes, two series of changes are seen to take 
place in the brain. Firstly there is an increase in the 
number of elements involving a growth of the brain, and 
secondly a differentiation of form and a tendency to the 
formation of groups of cells or “centres.” In a_post- 
larval specimen, 4°5 mm. long, the brain is ‘11 min. long, 
07 mm. wide and (05 mm. deep. Im a worm 75 mm. 
long the brain has about twice these dimensions (length 
‘2 mm.), and in another 17°5 mm. long the length of the 
brain has again doubled (‘4 mm.). Beyond this point the 
rate of growth is much slower, and in a specimen 10 inches 
(250 mm.) long, the brain is only ‘9 mm. in length. The 
description of the minute structure of the braim given 
just above is drawn from specimens about 60 mm. in 
length, in which the brain is a little more than half a 
