50 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Molluscoida. 
a. Tunicata. 
Origin of Peribranchial Cavity in Ascidian Embryos.* — Dr. 0. 
Seeliger has investigated Clavelina and Ciona , and concludes that in the 
embryo of Ascidians the whole wall of the peribranchial cavity is 
formed from ectoderm. In the development of buds, however, the cavity 
is formed from two endodermic diverticula. The dilemma is immediately 
suggested, Are the two cavities not homologous, or is an organ of ecto- 
dermic origin homologous with one derived from endoderm ? Dr. 
Seeliger accepts the second position, and suggests that when budding 
began in the phylogeny of Ascidians the ectoderm was too highly 
differentiated to be available for the formation of a peribranchial cavity. 
So also in regard to the nervous system of the bud, which the author 
(with Kowalevsky and Hjort) believes to be of endodermic origin. In 
fact there is little parallelism between the development of the ovum and 
the development of the bud. 
According to the author, the exhalent aperture and cloaca did not 
arise from a special median invagination into which the two lateral 
peribranchial sacs secondarily open ; they owed their origin to a dorsal 
expansion of the two peribranchial vesicles, which resulted in a union of 
the two apertures and the adjacent proximal portion. Finally, with 
special reference to Ciona intestinalis , Seeliger discusses the two different 
modes in which the branchial slits arise in ontogeny— two different 
modes which he endeavours to interpret as modifications of one. 
Formation of Mantle. f — Dr. 0. Seeliger has investigated this in 
Salpa democratica , Clavelina lepadiformis, and Oikopleura cophocerca , and 
confirms the conclusion recently stated by Kowalevsky and others that 
the mantle is in great part due to mesenchyme cells which migrate 
through the ectodermic epithelium. The latter has also a share in the 
formation, giving off secretory processes and also contributing cells. 
Indeed, if only the beginning of the formation be studied, it might be 
thought that the whole was due to ectoderm cells. The process is no 
cuticle-formation, for into the secreted substance cells and parts of cells 
pass and undergo modifications. 
British Tunicata. J — Prof. W. A. Herdman, returning to this branch 
of his studies after thirteen years, commences by remarking that with 
the fuller knowledge he now has of variations in the Tunicata, and after 
these years’ experience, he is inclined to think that he formerly laid too 
much stress upon minute structural characters, and described as new 
species several forms which it would be better to regard as varieties. A 
list of the Ascidians in 0. F. Muller’s 4 Zoologia Danica ’ is given, with 
the names that are now usually applied to the same forms; this is 
followed by a similar list of the species enumerated by Edward Forbes 
in Forbes and Hanley’s 4 British Mollusca,’ and by a consideration of 
the species of Alder and Hancock. With regard to the variability in 
the number of tentacles, the published records show differences extending 
* Zeitschr. f. wiss. Zool., lvi. (1893) pp. 365-401 (2 pis.). 
f Tom. cit., pp. 488-505 (1 pi.). 
X Journ. Linn. Soc., xxiv. (1893) pp. 431-53 (4 pis.). 
