ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
483 
as arising in the following way ; the mesodermal cells, while secreting 
contractile substance in the form of longitudinal muscular fibrils on 
their outer surface, or that turned towards the surface of the body, are 
on either side broken up by the developing longitudinal ridges of the 
ectoderm that form the lateral lines. The musculature and the lateral 
lines are developed at the same time. The muscle cells that line the 
coelom remain permanently epithelial and arranged in one layer, so that 
they may be regarded as epithelio-muscular cells. This is the same 
arrangement as the author has already described for the Echino- 
rhynchi. 
From their position it would be supposed that the excretory vessels, 
like the lateral lines, are of ectodermal origin, but, as a fact, they 
are mesodermal ; for they are derived from one or two cells of the coelom. 
In the stage in which the gonads consist of an undifferentiated meso- 
derm-cell, the excretory organ consists also of only one cell. In larvae 
taken from the body-cavity of Mugil cephalus and 0‘5 cm. long this 
cell was elongated, and was attached by one end in the region of the 
nerve-centre ; within the cell the excretory canal was already laid down 
as a simple narrow vessel. 
It appears to be true of all Nematodes that the excretory organ does 
not lie directly in the lateral ridges, but always in its connective tissue, 
which is only in very close connection with the ridges. These ridges 
serve as supporting tissue and generally exhibit a structure different 
from that of the skin, although they are directly continuous with it. 
The generative organs and the intestine lie in the coelom, and the 
latter has a wall consisting only of endoderm, and bounded on the 
side of the coelom by a membrana propria. There is no splanchnic layer 
of mesoderm in the wall of the intestine. This is explained by the fact 
that the mesoderm, at the end of its development, consists of an 
equally developed cellular layer, which lies directly on the ectoderm 
and is only interrupted by the lateral ridges. The coelom is now 
already present. This coelom is, therefore, not homologous with that 
of Annelids, but is a special formation, and, as some would say, an 
example of abbreviated development. 
Filaria tricuspis.* — Dr. von Linstow finds that under the name of 
Filaria attenuata two species have been described ; the true F. attenucita 
is found in Raptorial Birds, while that found in Crows is Fedschenko’s 
species F. tricuspis ; of the latter a descriptive account is given. There 
are several species of the genus which resemble it in having the head- 
end armed with a tridentate process — F. obtusa, F. pungens , F. ecaudata , 
and F. flabellata, and these are all found in Birds. The author draws 
attention to the points by which they may be distinguished. He con- 
cludes with some notes on blood-filariae and on the larval form. 
y. Platyhelminthes. 
Tetrastemma lacustris [e]-t — M. G. du Plessis describes a new species 
of Nemertine from the Lake of Geneva. It is at once distinguished by 
the possession of large oval concretions in the skin, which are highly 
* Arch. f. Naturgesch., lvii. (1891) pp. 292-305 (1 pi.), 
t Bull. Soc. Vaud., xxviii. (1892) pp. 43-8 (1 pi.). 
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