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2. The material examined. 
For my investigations I have used representatives of different 
groups of invertebrates as also vertehrates, I have chosen classes as 
little related to each other as possible, partly, in order that if I found 
uniformity in these classes as to the histological structure, the ob- 
servations might be assumed to be of general importance to the 
whole animal kingdom, excluding those lower classes which have 
not, yet, got a relatively more developed nervous system. The 
Echinoderms and Coelenterates I will pay no attention to in this 
paper, as I intend to keep them for a separate paper. 
As a representative of the lower MoUuscs I have chosen Patella 
vulgata, it being a large species which I coiild obtain in abundance 
in the neighbourhood of Bergen. 
Of Cliætopodes I have examined different species of Nereis, as 
well as, also, species of other families, e. g. Leanira, Nephtys etc. 
Of OUgochætes I have investigated Liimlwicus agricola. 
Of Crustaceans I have particularly investigated the Homarus vul- 
garis, as also, occasionally, Nephrops norvegicus and some Amphipods. 
Of Ascidians I have had for my investigations specimens of 
Phalusia mentula, Ph. oMiqtia, Ph. venosa, Pli, xjnmum, Ascidia 
scahra, Corella parallelogramma, Ciona intestinalis and species of 
Cyntliia. 
Of vertehrates, it is especially those from the lowest stage of 
evolution, viz. Amphioxiis lanceolaius and Myxine glufinosa to which 
I have paid attention. I have, however, also, examined other 
vertebrates e. g. Gadus morrhua, Tinca viilgaris, Bana temporaria, 
Mus musculus, Pelix domesticus, Homo sapiens and even Balenoptera 
rostrata. 
As, however, I intend to write a separate paper on the nervous 
system of the vertebrates, I will not pay much attention to these 
investigations on higher vertebrates in this paper, only so much may 
he said, however, that I, every where, have found the most beautiful 
uniformity in relation to most of the observations reported in this 
paper. 
As to the vertebrates, I shall confine myself to mention some 
observations on Amphioxus and Myxine. I got Amphioxus in 
abundance at Naples where, by Prof DoHRNs extraordinary gen- 
erosity, I was allowed to study at the zoological station, in the 
spring of last year in spite of Norway having no working table at 
the station. Quite recently I received a package from that station 
containing numerous specimens of Amphioxus most exellently pre- 
