— 57 — 
ganglion cells, i. e., such fibres as do not go to form the main 
longitudinal commissures.« As to the origin of the nerve-libres, and 
their relation to the dotted substance and the ganglion cells, it will, 
already, from what is just quoted, be seen that Packard does not 
quite agree with Leydig; 1. c. p. 5 he says for instance: »there 
is no doubt but that all the ganglion cells give rise to fibres, some 
of which at least pass directly through or above or around the 
myeloid substance of the cerebral lobes and form the commissures 
(i. e., the transverse as well as the longitudinal ones). This indepen- 
dence of the myeloid substance appears to be more general in the 
Assellidæ, at least this we would infer from Leydig's statements previ- 
ously quoted. « Packard even thinks it to be »little doubt but that in 
all Arthropoda certain nerve-fibres arising in the pro cerebral lobes 
(from ganglion cells) pass uninterruptedly to the last ventral ganglion « 
(i. e., through the whole central nervous system).^) A great many 
peripheric nerve-fibres, he supposes, however, to originate in the way 
which Leydig has indicated; he says that in the » myeloid substance « 
a great many processes from ganglion cells become » broken up 
into a tangled mass of fibrillæ, which unite finally to form the fibres 
constituting the nerves of the appendages.« Whether he supposes 
all peripheric nerve-fibres to originate in this way, or not, I have not 
understood from his description. 
Fraipont (1884) aftbrds some interesting informations regard- 
ing the histology of the nervous system of the ArcheanneUds. 
Judging from his description, I think there is no doubt, but that 
the central nervous system of these primary Annulates has a 
histological structure which is, principally, of quite the same type 
as that of the central nervous system of higher Annulates, 
Arthropods and other invertebrates, to be treated of in this 
paper. He distinguishes between two constituents, an external 
layer of ganglion cells and a central fibrillar mass ; for the 
latter he uses various designations : >^ substance fibro-nerveuses^-, 
»substance ponctuée«, » substance nerveuse fibro-ponctuée«, >aTiass 
finement ponctuée et fibrillaire« etc. It is evident that he sup- 
poses this mass to consist of a web of slender fibrillæ; 1. c. 1884 
p. 267 he says, for instance: »Figure 5 (pl. XI), on peut voir au 
milieu des cellules ganglionnaires les sections trausversales ou obliques 
As will be seen in my niemoir on Myzostoma (1885), p. 35 & 75, I 
have expressed myself in very similar terms upon this subject, and have said that 
I believed similar longitudinal nerve-fibres etc. to be generally present in Annelid^ 
and Arthropods. 
