On the Ancestral Form of the Chordata.

364 A. A. W. HUBREOHT
rically placed and of a general internal metamerisation,
but differing from Gunda in such important respects as the
presence of the forerunners, both of the hypophysis and of the
notochord, two structures no trace of which is found in the
salt-water Triclades. Such Platyelminths must needs
have resembled the present Nemertines more than
anything else.
Here the important question at once thrusts itself upon us:
Has the formation of a ecelom already been arrived at in the Nemertines
or not? i.e. have these animals a body-cavity developed
out of and separated from the primitive digestive tract or not ?
Although I have formerly, when attempts were made to bring
theNemertines under the so-called Parenchymatous Flat-worms,
combated those attempts, and endeavoured to show that the
regular arrangement of digestive and generative caeca, the development
of muscular septa between them, &c, went contrary to
it, yet now that our ideas about the significance of a true bodycavity
as an ultimate derivate of the archenteron have of late
years gained so considerably in clearness and definition, I
should hesitate to affirm that any such body-cavity is present
in Nemertines, and would be inclined to answer the question
proposed above negatively.
Both in the more highly, differentiated Hoplonemertini and
in the more primitive Schizo- and Palseonemertini, I have
met with numerous instances in which all the space which
remained free between the muscular body-wall on. the one
hand, and the intestinal, generative, proboscidian, and circulatory
cavities on the other, was one unbroken mass of connective
tissue.
Sometimes, more especially around the oesophagus, occurred
what were apparently fissures and cavities in this tissue. They
were not lined by an epithelium (are perhaps in communication
with the vascular system ?), and could best be compared
to a true Schizoccelom (Huxley), i.e. fissures in a mesoblastic
tissue.
All this makes me very much inclined to look upon the
alimentary diverticula of the Nemertines in the same light as