2S 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
locomotion. The third or pallial segment is formed of two rows of 
cells which are entirely covered by fine cilia. In a larva of 100 hours 
three imbricated spicules are to be seen on either side of the ventral line, 
still inclosed in their mother-cells. The spicules increase in number. 
The conical body elongates rapidly and becomes curved on its ventral 
surface, while the mantle is gradually reduced, and the embryo falls to 
the bottom, as the ciliated corona is unable longer to support it in the 
fluid. 
Only one of the author’s embryos passed safely through the critical 
period of metamorphosis, which is on the seventh day. This change 
consists in the casting off of almost the whole of the external envelope 
of the larva, that is to say, of the cells of the velum and the two rows 
that form the pallial lobes. Seven dorsal calcareous and slightly imbri- 
cated plates were observed in the surviving embryo. 
Till the time of metamorphosis the larva has no mouth, and the 
endoderm forms a solid mass flanked on either side by solid rows of 
mesoderm, the origin of which has not yet been made out. 
To sum up, the mode of segmentation is almost identical with that of 
Dentalium and certain I.amellibranchs ; the mouthless larva, formed of 
three segments, has no known analogue, except among the Brachiopoda ; 
the loss of the greater part of the ectoderm has been noted in Poly- 
gordius, and the tegumentary investment of the young Solenogastrid 
closely recalls that of young Chitons. 
5. LameUibranchiata. 
Primitive Structure of Kidney of Lamellibranchs.* — Dr. P. Pelse- 
neer points out that it is the generally received doctrine that the struc- 
ture of the renal organ of Lamellibranchs does not ally them to the 
lowest, but to the more highly developed representatives of the Proso- 
branchiata. This statement, however, is made on the results of the 
investigation of very specialized forms. When the more archaic repre- 
sentatives of the group, such as the Nuculidm or Solenomyidm, are 
dissected, a very different arrangement is found to obtain. In them 
each kidney forms a sac which is folded on itself in such a way as to 
have its two ends more or less approximated and directed forwards ; one 
of these opens into the pericardium, and the other to the exterior. In 
no Protobranch does the sac extend as far backwards as the posterior 
adductor, and it does not communicate with its fellow. As to structure, 
the kidney has no internal fold or lamella, and no ramifications ; it is 
an absolutely simple sac, with a large lumen. Its inner wall is formed 
by a uniform epithelial investment, extending from one extremity to the 
other, and having all its cells similar and secretory. This fact shows 
that, in the more specialized Lamellibranchiata, the terminal or postero- 
anterior branch of the kidney had not, as Rankin supposed, a primitively 
efferent function, but was originally secretory, like the whole of the 
gland. The arrangement which obtains in the Najidae, for example, 
when the secretory formation falls on the antero-posterior branch, is a 
specialization. 
From the point of view of structure there is a great resemblance 
between the protobranch Lamellibranchiata and the Fissurellidee, for the 
* Comples Rendus, cxi. (1890) pp. 583-4. 
