etrical met with in
Vertebrates.

The nervous system Semper found to develop in the buds of _Nais_ and
_Chaetogaster_ by an ectodermal thickening, just as in some Vertebrates.
The cerebral ganglion was formed by the ends of the nerve-cord growing
up round the oesophagus and fusing with the paired "sense-plates" which
develop from the ectoderm of the head. The cerebral ganglion is
accordingly only secondarily haemal in position, and there is no need
therefore to seek in Vertebrates for the homologue of the oesophageal
commissures of Annelids, as, for instance, Schneider did.

Since the mouth opens on the neural surface in Annelids and on the haemal
surface in Vertebrates, Semper considers that they cannot be equivalent
structures, and he finds the homologue of the Vertebrate mouth in a
little pit on the haemal surface of the head in the leech _Clepsine_ (also
in the true mouth of Turbellaria and the proboscis-opening in
Nemertines). The primitive Annelid mouth, however, does not appear in
the embryogeny of Vertebrates, for the great development of the brain
crowds it out of existence.

The homologues of the gill-slits Semper finds in two little canals in
the head of _Chaetogaster_, which open from the pharynx to the exterior.
In Sabellids he describes an elaborate system of gill-canals, with a
supporting cartilaginous framework which forms a real _Kiemenkorb_ or
gill-basket, comparable with that of Amphioxus.

Gill-slits, notochord, relation of nervous system, mesonephric tubules,
are thus common to Annelids and Vertebrates--what further proof could
one desire of the close relationship of these groups? Yet Semper enters
into refinements of comparison, seeing, for instance, in the lateral
portions of the ventral ganglia (Fig. 14, _sp. g._) the homologues of
the spinal ganglia of Vertebrates, and comparing the lateral line of
sense organs in Annelids with the lateral line in Anamnia.

He will not admit that Amphioxus and the Ascidians show a closer
resemblance to Vertebrates than his beloved Annelids. Amphioxus, he
thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids
the possession of a notochord, gill-slits, and a "dorsal" nervous
system, yet are further removed from Vertebrates than the latter by
reason of their lacking that essential characteristic of Vertebrates,
metameric segmentation.

Not content with establishing the unity of plan of Annelids, Arthropods,
and Vertebrates, Semper tries to