ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 37 



with the sensory nerves, arise the motor nerves. The bulk of the 

 memoir is devoted to the histological results. 



The central canal and the supporting elements. — The supporting ele- 

 ments of the central nervous system consist (a) of the conical epithelial 

 cells which line the central canal and (6) of fibres. The apices of the 

 conical cells are directed outwards and continued into threads, which 

 either penetrate tne central nervous system undivided and are inserted 

 in the connective tissue sheath, or else ramify. In varying degrees the 

 epithelial cells lose themselves in the fibres. Sometimes only the 

 nucleus is left — the " supporting-fibre-nuclei." Along with the strong 

 undivided processes of the conical cells, other nuclei, probably nervous, 

 occur. Few conical epithelial cells occur in the dorsal portion of the 

 central canal-wall; the fibrous upbreaking is less marked from above 

 downwards. With these results the observations of other investigators 

 are then contrasted. 



The nervous elements of the nerve-cord. — The nerve-fibres, composing 

 the greater part of the nerve-cord, and forming a ring round the 

 ganglion-cells, are without medulla and of very varied strength. Very 

 thin fibres predominate in the dorsal portion ; those in the ventral half 

 are thicker and more distant. Giant nerve-fibres among the latter are 

 found in the same position all along the cord. The strongest, lying 

 ventrally to the central canal, is unpaired ; the others lie in three lateral 

 paired groups. The intimate structure seems to consist of fibrils of 

 extreme fineness, but of this only a trace was to be seen in the giant 

 fibres. The fibres lie imbedded in the fine meshwork formed by the 

 supporting elements ; few lateral branches are given off, but a deceptive 

 appearance of this is produced by the supporting elements. Bifurcation, 

 however, frequently occurs. 



The ganglion-cells vary greatly in size ; small, medium, and giant 

 forms occur here also. Among the small cells, unipolar and bipolar 

 forms predominate. They lie for the most part beside the epithelial 

 cells, and are very like them. The medium cells Include all forms. 

 The giant ganglion-cells are exclusively multipolar. They always lie at 

 the boundary between the dorsal and median third of the central canal. 

 Their processes stretch right and left into either half of the cord. 

 They are relatively few in number. The processes of the giant cells 

 are of two kinds, one set passing into fine nerve-fibres, the others — one 

 from each cell — retain a large size as the giant nerve-fibres already 

 noted. The most anterior ganglion cell gives origin to the median 

 giant fibre — the largest of all — and to seven diminishing fibres only 

 traceable for a short distance. The processes of the other ganglion-cells 

 are described at length, and again the results of other investigators are 

 brought into contrast with the author's. 



The hrain. — The central canal expands in front of the origin of the 

 second pair of sensory nerves. A many-layered sheath of very closely 

 packed cells and nuclei surrounds it. Some look like the typical conical 

 epithelial cells, in their original position, or displaced outwards. 

 Numerous cell-less nuclei (nerve-nuclei) occur ; at the end of the nerve- 

 cord they occur not only on the epithelial layer, but among the nerve- 

 fibres, especially on the dorsal surface, and extend in part to the sensory 

 nerves. In the epithelium of the ventricle, the supporting and the 

 nervous fibres are hardly distinguishable from one another. Eound the 

 pigment-spot is a thick layer of small dark nuclei, passing posteriorly 



