W. B. Coe — Aiiatomy of Cerebratulus lacteus. 501 



abundant in the inner and anterior borders of the dorsal brain-lobes, 

 on all sides of the ventral lobes, as well as in the lateral cords and 

 in the ventral commissure of the brain-lobes. These cells are very 

 large in comparison with those of the other types, measuring in 

 mounted specimens .025"""^ by .015°"^. Each cell is pear-shaped and 

 is provided with a large, spherical nucleus near the larger end. The 

 smaller end is continued into a process which, although large and 

 conspicuous, can be traced but a little way from the cell. These 

 cells mostly lie nearer the periphery of the brain than those of the 

 other types of cells. Surrounding each cell is a delicate sheath of 

 connective tissue. 



The fibrous core (Plate X, fig. 6 ; Plate XV, fig. 3,/c) is made up 

 of exceedingly fine and delicate fibres. In the brain this core swells 

 out in the dorsal and ventral lobes. The cores of the two sides are 

 connected by both commissures. Each dorsal lobe shows a partial 

 division into an upper lobule which ends free posteriorly, and an 

 under lobule which continues into the side-organ. Among the 

 fibres occurs a delicate, supporting connective tissue with a few 

 oval nuclei. Outside the fibrous core is a delicate inner sheath of 

 connective tissue, or neurilemma (Plate XV, fig. 3 ; Plate X, fig. 6, 

 C5), through which the fibres from the ganglion-cells pass in bundles 

 to spread out among the fibres of the core. Supporting the ganglion- 

 cells is a loose network of connective tissue (c) which, in certain 

 parts of the brain, notably on the inner borders of the ventral lobes 

 and the upper borders of the dorsal lobes, occupies a considerable 

 area which is devoid of ganglion-cells. Surrounding the ganglion- 

 layers is an outer sheet of connective tissue [cs') much thicker than 

 the inner neurilemma and more or less broken up into broad bundles 

 of fibres. 



The lateral cords (Plate X, fig. 5) are composed of a central mass 

 of fibrous tissue {fc) surrounded by a sheath of connective tissue 

 [cs) much thicker than the inner neurilemma of the brain. Delicate 

 nucleated fibres from this sheath run in all directions amons^ the 

 nerve-fibres, and often cross from one side of the core to the other, 

 so that a few connective-tissue nuclei are often met with in the mid- 

 dle of the core. Outside this inner sheath, or neurilemma, the gan- 

 glion-cells [gc) are arranged in a large mass above and one below the 

 fibrous core. These cells are mostly of the second type with a few 

 of the third type towards the periphery. On the outer and inner 

 borders of the cord no ganglion-cells occur, the outer border having 

 a broad band of loose connective tissue (c) with a few longitudinal 



