SEA-FISHERIES LABORATORY. 255 



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 lies on the 

 inner side of the nuchal organ just below its sensory 

 epithelium (figs. 34, 47). The posterior lobes are separated 

 by a coelomic 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 mm. long, 

 •07 mm. wide and '05 mm. deep. In a worm 7'5 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 brain given 

 just above is drawn from specimens about 60 mm. in 

 length, in which the brain is a little more than half a 



