672 SUMMARY OF CURRENT RESEARCHES RELATING TO 
last is made the representative of a new family, the Polyclinopsidse, 
which appear to occupy an isolated systematic position ; while most 
allied to the Polyclinidae, on account of the persons being divided into 
three regions, it is distinguished by having the gonads separate, and the 
testes not composed of racemose follicles. From the Distomidse it is 
separated by the complete absence of ectodermal appendages and a stalk. 
With some resemblances they are distinguished by, inter alia , the pre- 
sence of a post-abdomen and the regularity of the cormidia. 
All the species are described in detail. 
Perophora annectens.* — In introducing this new species of American 
Tunicates, Mr. W. E. Ritter remarks that, though he believes it to be a 
Perophora, its chief characteristic would, with some of the schemes of 
Tunicate classification, lead to its being placed in a different suborder ; 
this characteristic is the fact that in very many, though not in all, of 
the colonies the ascidiozooids are as completely imbedded in a common 
test as they are in Botryllus and Goodsiria. The author is of opinion 
that the distinction between “ simple ” and “ compound” Ascidians, the 
importance of which has diminished in the same ratio that our know- 
ledge of the group has increased, is reduced to nil by the discovery of 
this form ; so far, that is, as its value in determining affinities is con- 
cerned. A detailed account is given of this new form. 
Arthropoda. 
Germinal Layers, Yolk Cells, and Embryonic Membranes of 
Arthropoda. | — Herr J. Wagner has been led from his embryological 
studies on mites and Mysis to some general conclusions. He begins by 
noticiDg the varied effects of the amount of yolk on cleavage and on the 
formation of layers. Thus the origin of the inner layer (meso-endoderm) 
by immigration, e. g. in Crustacea, is a secondary result. Comparing 
the early stages in the development of the Arthropod types, he concludes 
that Myriopods and Insects have been derived from Annelid-like ancestral 
forms with elongated blastopores, Crustaceans and Arachnoids from 
forms with round blastopores. Among Arachnoids, the mites show an 
original distinction between endoderm and mesoderm elements, and this 
is primitive. The yolk-cells of Crustaceans, Myriopods, and Insects 
represent a special type of vitellophagous elements, not derivable from 
ancestral forms $ but those of Arachnoids are independent of those of 
other Arthropods, e.g. in showing a much earlier differentiation. The 
embryonic envelopes probably originated independently in the different 
groups : the insinking of the embryonic streak or imaginal plate depends 
on its sharp differentiation into covering cells and embryocytes. In 
fact, that invagination in some form must follow such differentiation is 
a general fact of development. The insinking of the embryonic streak 
leads to the formation of the envelopes ; and they are subsequently cast 
off in consequence of the relatively more rapid growth of the embryonic 
portion. 
* Proc. Cal. Acad. Sci., iv. (1893) pp. 37-85 (3 pis.). 
f Biol. Centralbl., xiv. (1894) pp. 361-75. 
