ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
741 
B. Nemathelmintlies. 
Structure of Nemathelminthes .* — Dr. 0. Hamann calls attention to 
tlie paedogenesis not only of Echinorhynchus clavseceps but also Ech. agilis. 
Both persist and are mature in a larval state. Like Archigetes Sieboldi, 
they have arisen by “ phylo-psedogenie.” The author would interpret 
many forms, even Amphioxus, in a similar way, as larval stages which 
have become sexually mature. Four species of Echinorhynchus which 
Molin described as distinct, viz. Ech. crassatus , Jlavus, de Visianii , and 
solitarius , are really one. Hamann was fortunate enough to find numerous 
specimens of the genus Lecanocephalus, of which little has hitherto been 
known. It has only one longitudinal vessel, situated along the right 
lateral line in the anterior half of the body. This vessel opens to the 
exterior under the nerve-ring, and communicates posteriorly with the 
body-cavity. Some preliminary notes on the various structures of Nema- 
todes are communicated. 
Monograph on Acanthocephala.f — Dr. O. Hamann begins his mono- 
graph with an account of the maturation and segmentation of the ova in 
Echinorhynchus acus. Among the segmenting ova in the body-cavity 
lie ensheathed egg-balls which result from the disruption of the ovary. 
While the egg-cells are still in this stage, two polar bodies are formed. 
The precise moment of fertilization remains unknown; the sperma- 
tozoa are sometimes found in the body-cavity of the female ; it is certain 
that in Ech. acus they penetrate the membrane of the egg-ball, for within 
this the first two stages of division occur. The two cells which result 
from the first division are unequal, the larger being towards the pole which 
bears the polar bodies ; but on the whole the segmentation is regular. 
The central cells are always richer in chromatin than the peripheral. 
There is a triple sheath round the whole mass. The epiblast is never 
without nuclei, though these are poor in chromatin ; a few giant nuclei 
appear at an early stage at the posterior end ; in the larva the epiblast 
is a syncytium. In Ech. hseruca no polar bodies were observed; the 
segmentation is at first very irregular. The stage with an epiblast of 
several layers all poor in chromatin, and with a hypoblast represented 
by an internal mass of cells whose nuclei are rich in chromatin, is 
regarded as a gastrula. The larval forms Ech. proteus and Ech. polymorphus 
are then described. Hamann regards Ech. clavseceps Zed. as a species 
which has arisen by paedogenesis, for it is sexually mature at a stage 
which corresponds to a comparatively early one in most other Acantho- 
cephala. 
In the second part of his monograph the author describes the 
histology and organogeny. The larval state of the skin — a syncytium 
with a few giant nuclei and direct division — persists in Ech. clavseceps. 
Tho lemnisci act like the ampullae of starfishes, helping in the extrusion 
of the proboscis, serving as a reservoir for the lacunar fluid. The 
musculature and the proboscis, the ganglion of the proboscis sheath, 
and the peripheral system, the gonads and their associated ducts are 
all described in detail. Under the title Ech. proteus Westr. von Diesing 
* SB. K. Preuss. Akad. d. Wiss. (1891) pp. 57-61. 
f Jenaische Zeitschr. f. Nat., xxv. (1890) pp. 113-231 (10 pis. and 4 figs.). 
