590 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Gastrulation takes place by invagination. In the development of the 
prototroch the egg shows a relation both to Annelids and to Gastropods. 
By a detailed comparison with the cells of the Annelid egg, the author 
endeavours to show that the head of the Annelid corresponds to that 
part of the Chiton which constitutes the proboscis, the first valve of the 
shell, and the mantle-furrow with the eyes. Further, the distinct traces 
of radial symmetry in the embryo lead him to the conclusion that the 
ancestral trochophore was a quadrilaterally symmetrical organism whose 
principal axis corresponded to that of the gastrula, and that the shifting 
of this axis is secondary. The radial cleavage of the egg is due to the 
influence of the radial symmetry of the ancestral trochophore. The 
resemblances between Annelid and Chiton trochophores are such as to 
make it impossible to doubt that they are derived from a common 
ancestor. 
S. Lamellibranchiata. 
Contributions to our Knowledge of the Protobranchia.* — Dr. G. 
A. Drew has studied Yoldia limatula Say, Nucula delphinodonta Mighels, 
and Nucula proxima Say, all found along the coast of Maine. A brief 
statement of the chief points may be cited. 
The mantle of Y. limatula has two pairs of sense-organs, an unpaired 
siphonal tentacle, and a fringe of marginal tentacles, all sensitive to 
mechanical stimulation. The foot is highly modified for burrowing ; 
the palp-appendages are very active in collecting food ; the gills of 
Yoldia are used for pumping water, and those of Nucula may function 
in the same way to a less extent. The genital ducts join the outer and 
not the inner ends of the excretory organs. The otocysts of T. limatula 
are not connected with the surface by canals, for these seem to degene- 
rate in the adults. 
As to embryology, the embryos of N. delphinodonta are carried for 
many days in mucus-like cases attached to the posterior ends of the 
shells. The original surface cells seem to give rise to the tests, the 
cerebral ganglia, the apical plates, and to new ectoderm inside of the 
tests. The cerebral ganglia of Y. limatula are formed as invaginations 
from the surface. The otocysts of Y. limatula contain otoliths before 
the tests are cast. The test, at least part of the apical plate, and the 
stomodaeum to the position of the definitive mouth, are all cast away. 
It seems that typical molluscan larvae may have been developed from 
ancestors resembling the embryos of Yoldia and Nucula in form and 
structure. 
Swiss Fresh-water Molluscs.j — M. Georg Surbeck has made a study 
of the molluscan fauna of the Lake of Lucerne, which includes 22 litt ral 
forms, and one in deep water ( Pisidium clessini). Compared with that 
of other Swiss lakes, the molluscan fauna here is poor, and shows no 
local characteristics of moment ; it is most like that of the Lake of 
Constance and of some upper Bavarian lakes. Differences in size, shell- 
thickness, and abundance are noteworthy at different parts of the lake ; 
they seem, for the most part, to be due to the environmental differences. 
* Anat. Anzeig., xv. (1899) pp. 493-519 (21 figs.). See also Mem. Biol. Lab. 
Johns Hopkins, iv. (1899) No. 3. 
t Rev. Suisse Zool., vi. (1899) pp. 429-556 (2 pis. and map). 
