The visceral ganglion of Pecten, etc. 
9 
in the case of the laxge cells of the dorso-central lobes the axones 
can be traced through the neuropil and the ganglion into the peripheral 
nerves, without the Intervention of a nerve net. The structure of the 
large ganghon cells is as follows. The celi is membraneless, and there 
is no distinct division of the cytoplasm into a marginai ectoplasm and 
a centrai endoplasm. The axone is however more hyaline and the 
celi has a narrow more hyaline margin, the outer boundary of which is 
not easily seen owing to the enclosing neuroglia fibres and its weaker 
affinity for stains. The perinuclear cytoplasm or that of the centre of 
the celi, if the nucleus is situated eccentrically, often contains a large 
mass of granulös of an orange brown colour (fig. 11 Pg), which gives rise 
to the pigmented appearance of the ganghon. 
Kawitz after a detailed comparison, states (p. 15) that the ganglia 
of siphonate lamellibranchs are not pigmented. "In den Zellen findet 
sich meistens, die Siphoniata und die Muscheln mit hellgefärbter Schale 
ausgenommen, ein orangefarbenes Pigment." This rule does not hold 
good for the few siphonates that I have examined. The ganglia of Mya 
are pigmented and pigment is to be found also in Venus, Astarte, etc. 
The small cells do not appear to contain the pigment, and the depth 
of colour in the ganglion of Pecten, varies also in individuals. In some 
sections of the visceral ganglion of Venus casina, traces of the pigment 
are to be observed between the ganghon cells. Probably, as suggested 
by AsHWORTH (1909, pag. 499) it represents insoluble products of meta- 
bolism. 
After staining with Held's modification of Nissl's Stain, (see Arch. 
Anat. Phys., Anat. Abth. 1907) numerous blue granulös are distinctly 
visible in the cytoplasm of the ganglion cells (fig. 14). These chromo- 
phil structures are scattered throughout the cells, but are more frequent 
near the centre; no regulär arrangement was observed. 
The Nucleus. The nucleus is large and spherical in shape, and 
in the large ganglion cells attains a dianieter of 13 ^, about half that 
of the cell. Its position is usually central but it may be much nearer 
the axone end, or the other end of the cell. A distinct nuclear membrane 
is present (fig. 11 N,m). The chromatin which is on the whole rather 
scanty, occurs as small granulös arranged in the form of a network. An 
obvious nucleolus is present (fig. 11 Nu) and on rare occasions two have 
been seen. With methyl blue-eosin the nucleolus stains an intense red, 
whilst with Held's stain its colour has something in common with the 
chromophil granulös, but is a deep purple against the bright blue of the 
latter. 
