embryos of other worms ; for instance, the leeches. Cer- 
tainly the development of the structure in question is so 
different in the two cases, as Leydig has rightly remarked, 
that one can only trace out the resemblance with the primi- 
tive streak of Arthropods through the similarity of the 
metamorphosis. The strange and unlooked-for thing con- 
sists in this, that the primitive streak develops itself in the 
leeches on an embryo, which all the while is leading an 
individual life, whilst it is in the other case the still struc- 
tureless yelk which gives rise to this appearance as the first 
indication of embryonal structure. That the Chaetopods 
should resemble the Bdellidae in this respect is no doubt 
new, but it is scarcely surprising. This fact first became 
known to the authors through the observation of the embryos 
of Oligochaeta. These last present a much closer similarity 
with the embryos of leeches than with the developing-stages 
of Polycliaeta, a fact which, in view of the many points of 
contact between Oligochaeta and Bdellidae, is not at all 
extraordinary. 
In all Polycliaet embryos the ventral baud makes its 
appearance as a remarkable thickening of the so-called 
“ animal layer ” quite unmistakably. But quite as little 
as in the leeches does this occur in the very first process of 
embryo-formation ; for this thickening often develops itself 
first when the embryo has begun to lead a free life. At 
first it is proportionately inconspicuous, but very rapidly it 
acquires thickness. Much later the ganglia of the ventral 
cord commence to be differentiated in it. The remarkable 
thickness of the ventral side has been already described, and 
figured in many embryos and larvae, but always without any 
reference to the ventral band of the Arthropods and Bdellidae. 
The difference, then, in the development of Annelids, accord- 
ing as there is the presence or absence of a so-called Bauch- 
streif (ventral band), ceases to have any importance. 
After the introductory remarks, of which we have given 
the substance above, MM. Claparede and Mecznikow pro- 
ceed to describe the larvae of Spio fuliginosus, Clprd. ; Spio 
Mecznikowianus , Clprd. ; Nerine Cirratulus, Della Chiaje ; 
Poly dor a ( Leucodore , Johnst.) ; Tclepsavus Costarum, Clprd. ; 
Pliyllochcetoptcrus socialis, Clprd. ; Lumbriconereis sj). ; 
Ophryotrochapuerilis, Clprd. ; Staurocephalus Chiaji, Clprd. ; 
Nephthys scolopendroides, Della Chiaje; Phyllodoce sp 
Capitdla capitata, F abr. ; Cirratulus sp. ; Audouinia Jiligera, 
Della Chiaje; Tercbella Meckellii, Del. Ch. ; Dasychone 
lucullana , Del. Ch. ; Spirorbis Pagenstecheri, Qtrfg. ; Pilco- 
laria militaris, Clprd. 
