500 W. H. Coe — Ajiatomy of Cerehratulus lacteus. 



cords are constituted consist of a central fibrous core covered with a 

 delicate sheath of connective tissue, or neurilemma ; a thick layer of 

 ganglion cells with masses of connective tissue peripherally ; and an 

 outer sheath of connective tissue covering the whole. 



Burger (2) finds in some species of Nemerteans four types of 

 ganglion-cells which differ more or less in size and shape and in their 

 relations with each other. Of these four types of cells three only 

 are met with in this species ; the fourth type, the " neurochord- 

 cells," not being present, or at least they have not been discovered. 

 All ganglion-cells appear to be unipolar, and are surrounded by a 

 loose sheath of connective tissue. 



Th.Q first type of ganglion-cells (Plate XV, fig. I^gc^) seems to be 

 characteristic of the dorsal brain-lobes. They are conspicuous from 

 their small, deeply-stained nuclei which in the sections are packed so 

 closely together that it would seem that very little space could be 

 left between them for the cell-protoplasm which is seen with diffi- 

 culty, although from it a slender process can sometimes be made out. 

 The cells occur most abundantly in the outer and posterior borders 

 of the dorsal brain-lobes, but are also found in the dorsal commissures 

 and among the tissues of the head, surrounding the duct leading to 

 the side-organs. There are large masses of such cells in the side- 

 organs themselves. In the dorsal brain-lobes these cells are packed 

 together in large masses above and below the fibrous core. Cells of 

 the third type also occur abundantly in the inner and anterior bor- 

 ders of the dorsal lobes ; otherwise these lobes contain only those of 

 the first type. As the majority of the organs of special sense, includ- 

 ing the side-organs (and eyes in those species in which they are pres- 

 ent), are supplied with nerves from the dorsal lobes, there is evidence 

 that the cells of the first type are sensory cells and that the cephalic 

 nerves are largely sensory in their function. 



The second type of ganglion-cells (Plate XV, figs. 3, 6) is charac- 

 teristic of the ventral brain-lobes and lateral cords. They occur 

 in large, radial clusters with their processes converging towards the 

 fibrous core into which bundles of fibres from these cells pass at 

 certain points. After entering the fibrous core the fibres spread out 

 and are lost to sight among the other fibres of the core. In these 

 cells, also, the nucleus is very large in porportion to the size of the 

 cell, which, however, is much larger than that of the first type, 

 although only about half as large as a cell of the third type, measur- 

 ing about .OOS"""^ by .OlS™'". 



Ganglion-cells of the third type (Plate XV, figs. 3, 7, gc^) are 



