some neio Siiecies o/DrawiJa. 513 



apparently Avithout any axons, which I have not succeeded iu 

 making out. There are four of them in each segmental 

 division of the cord, two in front and two behind the nerves, 

 and their position iu the cord can be better understood 

 by reference to the figure. They are regularly repeated 

 throughout, and are situated on the central bundles of giant 

 fibres on either side of the median clear space. In regard 

 to the details of structure of these giant cells, I might 

 mention that the large, centrally placed nucleus beat's a 

 deeply staining nucleolus. The nuclear membrane is thick 

 and clearly defined, and the chromatin granules are strung 

 out on the linin fibrils. The cytoplasm of these huge 

 neurons is full of tigroid or Nissl bodies, comprising masses 

 of nenrochondrian granules in addition to less deeply- 

 staining granules, mostly aggregated near the periphery of 

 tlie cell. These latter, perhaps, represent the disintegrating 

 ])articles of reserve food-material. The usual network of 

 fibrils is also present, but apparently without any implan- 

 tation cone or axons. In line with these larger cells are 

 others which are indifferent in their structure and are fibro- 

 balsts. They occur also in the marginal portions of the 

 cord. The true neurons are of two kinds, those with one 

 only and others with two nucleoli. They differ from the 

 giant cells in the possession of nerve-fibres, which lead 

 out from them. Each axon immediately after emergence 

 divides into two parts, the neurite and the dendrite. Even 

 without teasing the nerve, it is easy to discover that there 

 are eight of these neurons in each side of the ganglia or the 

 jjoint from which nerves are given off, and I have not 

 noticed any neurite or dendrite crossing over from one side 

 to the other. This is, again, a primitive organisation, and 

 shows that each half of the cord is composed of self-con- 

 tained ganglionic nerve-units. In regard to the structure 

 of the nucleus and the cytoplasm, these axon-bearing 

 neurons and others which occur always in pairs resemble 

 the giant cells. Though there is no experimental or direct 

 structural evidence to prcjve that the two kinds of axon- 

 bearing cells are physiologically different, it is at least 

 certain that the cells with double nucleoli cannot be function- 

 ally identical with those with a single nucleolus. In the 

 prostomium it is possible to trace the neurites, both per- 

 ceptory and distributory ones, from their source or origin to 

 their insertion or ending, with breaks in the interval where 

 the stain is unable to pick them. Tlie fibres arising from 

 the cells with double nucleolus are with difficulty traceable 

 to the epithelial or sensory cells and tactile organs, and 

 Ann. ct Mag. N. Hist. Ser. 9. Vol. viii. 33 



