ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
27 
The Dorsal Appendages of Tethys leporina.*— Prof. C. Parona has 
observed the discharge or autotomy and their rapid regeneration ot the 
remarkable dorsal appendages ( Phcenicurus ) of this mollusc — two facts 
which seem hitherto to have escaped observation. 
Molluscoida. 
a. Tunicata. 
Embryology of Pyrosomidae.f — Prof. W. Salensky completes his 
account of the intricate life-history of Pyrosomidm, describing the for- 
mation of the tetrazoid embryo and the development of the ascidiozoids. 
After an outline of the changes of shape which the embryo undergoes, 
he gives a detailed account of the derivatives of each germinal layer. 
fa) Ectodermic. The tunic is due in part to a great migration of 
mesenchyme cells into the space between the outer surface of the cyatho- 
zoid and the follicle wall, and in part to a secretion of cellulose from 
ectodermic cells. The nervous system of the ascidiozoid (and also of tho 
cyathozoid) arises from an ectodermic invagination with a very small 
cavity. From the nervous vesicle two tubular processes grow downwards ; 
the ciliated groove of the cyathozoid is formed from the central system ; 
but in the ascidiozoids there are two ciliated grooves, the first due to tho 
nervous system, that which replaces it to an evagination of the primitive 
gut. In the ascidiozoids there are two tcntacle-liko sensory structures 
exactly corresponding to those in Salpa. 
The peribranchial tubes of the cyathozoid and of the first four 
ascidiozoids are of ectodermic origin ; their development and that of the 
cloaca are described. 
(b) Mesodermic. From the mesoderm arise the pericardium and the 
heart, the elmoblast, the problematic organs — elongated and pisiform 
masses — of which the former arc due to tho calymmocytes, the muscles 
of the body, and the bud-stock mesoderm which is partly of independent 
origin and partly a continuation of the pericardium. All these aro 
described at length. 
(c) Endodermic. In the later stages in the development of the germi- 
nal disc, the posterior part of the gut-cavity is distinguishable from the 
anterior part by the presence of the endostyle ; the anterior part remains 
in the cyathozoid and represents the gut-cavity of the nurse-generation ; 
the posterior part passes into the stolon as the rudiment of the gut in 
the ascidiozoids. The development of the two parts is described. 
Salensky then explains the facts which lead him to come to tho 
following conclusions in regard to the development of the ovary. In all 
Tunicates tho originally liomoblastic rudiment of the ovary is differen- 
tiated into follicle-cells and ova. The mono-ovular ovary of Salpa and 
Pyrosoma is derived ontogenetically (and surely also phylogenetically) 
from a poly-ovular type, and is homologous with the ovary of Ascidians. 
The kalymmocytes, which appear between the ovum and its follicle, are 
in Salpa and Pyrosoma of extra-ovular origin, are in fact migrant 
follicle-cells. The homology between ova and follicle- or test-cells is 
proved by their origin from the same rudiment and by the modifiability 
* Zool. Anzeig., xiv. (1891) pp. 293-5. 
t Zool. Jahrb., v. (1891) pp. 1-98 (8 pis.). 
