THE ANATOMY OF SOALI BREGMA INFLATUM. 265 



the dorsal and ventral portions of the brain^ and there is a 

 group of larger nerve-cells jnst below their point of origin. 

 The posterior brain-lobes consist of nearly equal parts of 

 cellular and fibrous elements. The fibrous matter is covered 

 internally by a thin layer of cells, but externally has a thick 

 coating of ganglion cells, which are closely applied to the 

 nuchal epithelium. This mass of cells forms a large gan- 

 glionic centre. The posterior brain-lobes are broad in front 

 where they are fused with the anterior part of the brain, and 

 in this i-egion there are numerous comparatively large nerve- 

 cells, especially on the inner faces of these posterior lobes 

 abutting on the coelomic cavity. On tracing these lobes 

 backwards along the inner side of the nuchal organ, it is seen 

 that the cells decrease rapidly in quantity, and each lobe is 

 continued as a fibrous tract or nerve, which is accompanied 

 by only a very thin covering of cellular elements {N. Nuc, 

 fig. 15). This divides into two or three nerves near the 

 posterior end of the nuchal organ. The nerves lie between 

 the epithelium and the sheath of the organ. 



There is a little neurilemma on the dorsal and ventral faces 

 of the brain, from which strands pass inwards, supporting the 

 nervous elements. 



The above is a description of the brain of moderately 

 young specimens 13 to 14*8 mm. long. The average measure- 

 ments of the brain of five such specimens are "23 mm. long, 

 •22 mm. broad, and "IG mm. deep. As the animal increases 

 in size the brain not only grows in bulk but i;ndergoes con- 

 siderable changes in appearance. In a specimen of 56 mm. 

 long the brain is '35 mm. long, '5 mm. wide, and "35 mm. 

 deep. The fibrous portion of the brain in this specimen is 

 proportionately larger and much more complex, and the 

 neuroglia is better developed than in smaller specimens. 

 The nerve-cells, some of which are 30 fx in diametea*, are 

 aggregated into definite groups, separated by masses of 

 fibrous tissue. As in younger specimens the fibrous elements 

 ai'e chiefly internal, and are covered by the cells. At the 

 point of origin of the connectives the fibrous matter is ex- 



