The earliest Vermalia in which we first find this
independent vascular system are the Nemertina (Figure
2.244). As a rule, they have three parallel longitudinal
vessels connected by loops, a single dorsal vessel
above the gut and a pair of lateral vessels to the
right and left. In some of the Nemertina the
blood is already coloured, and the red colouring matter
is real haemoglobin, connected with elliptical discoid
cells, as in the Vertebrates. The further evolution
of this rudimentary vascular system can be gathered
from the class of the Annelids in which we find it
at various stages of development. First, a number
of transverse connections are formed between the dorsal
and ventral vessels, which pass round the gut ring-wise
(Figure 2.362). Other vessels grow into the body-wall
and ramify in order to convey blood to it. In
addition to the two large vessels of the middle plane
there are often two lateral vessels, one to the right
and one to the left; as, for instance, in the leech.
There are four of these parallel longitudinal vessels
in the Enteropneusts (Balanoglossus, Figure 2.245).
In these important Vermalia the foremost section of
the gut has already been converted into a gill-crate,
and the vascular arches that rise in the wall of this
from the ventral to the dorsal vessel have become
branchial vessels.
We have a further important advance in the Tunicates,
which we have recognised as the nearest blood-relatives
of our early vertebrate ancestors. Here we find
for the first time a real heart—i.e.
a
central organ of circulation, driving the blood into
the vessels by the regular contractions of its muscular
wall, it is of a very rudimentary character, a spindle-shaped
tube, passing at both ends into a principal vessel
(Figure 2.221). By its original position behind
the gill-crate, on ventral side of the Tunicates (sometimes
more, sometimes less, forward), the head shows clearly
that it has been formed by the local enlargement of
a section of the ventral vessel. We have already
noticed the remarkable alternation of the direction
of the blood stream, the heart driving it first from
one end, then from the other (Chapter 2.16).
This is very instructive, because in most of the worms
(even the Enteropneust) the blood in the dorsal vessel
travels from back to front, but in the Vertebrates
in the opposite direction. As the Ascidia-heart
alternates steadily from one direction to the other,
it shows us permanently, in a sense, the phylogenetic
transition from the earlier forward direction of the
dorsal current (in the worms) to the new backward direction
(in the Vertebrates).
(Figure 2.363. Head of a fish-embryo, with
rudimentary vascular system, from the left. dc Cuvier’s
duct (juncture of the anterior and posterior principal
veins), sv venous sinus (enlarged end of Cuvier’s
duct), a auricle, v ventricle, abr trunk of branchial
artery, s gill-clefts (arterial arches between), ad
aorta, c carotid artery, n nasal pit. (From Gegenbaur.)
