1885.] Affinities of Annelids to Vertebrates. 771 
segment of the trunk, and thus a stage is present where the seg- 
mental organs are connected by a common longitudinal duct. In 
the annelid this connection is lost. (though perhaps retained in 
a few), while each segmental organ acquires an external open- 
ing. In the vertebrate the primitive connection persists, and no 
external opening is developed for each segmental organ. 
The conclusion reached by Hatschek is that annelids and ver- 
tebrates have had a common segmented ancestor, which did not 
differ much from such a simple annelid as Polygordius. 
The three authorities now mentioned represent sufficiently the 
views of those finding more or less close relationship between 
the vertebrates and the annelids; there are others who regard 
this relationship as very remote, if existing at all, either placing 
other invertebrate forms nearer the vertebrates or separating this 
group from the annelids, from consideration simply of the funda- 
mental differences of the two groups. Among the latter may be 
reckoned Mr. Adam Sedgwick, whose ingenious hypothesis in- 
, cludes the question of the relationship of these two groups, 
though advanced with the much wider object of accounting for 
segmentation and the origin of the Triploblastica from ccel- 
enterate-like forms. Founded upon the behavior of the blasto- 
pore and upon the structure of the ccelenterates, the hypothesis 
calls for a coelenterate-like ancestor with mouth and anus part of 
one common opening to the digestive tract which bore diverticula 
on each side, indicating a segmentation, and communicating with 
each other as well as with the exterior; the nervous system 
formed a broad ring around the mouth-anus (Fig. 5). From this 
common hypothetical 
Of sore) j= pI] YY 
ancestor the annelids 
la a NMA T TT) 
it 
were derived by the 
Uf 
mouth and anus, by P ULL} yy 
the partial fusion of - 
pL LLL 
nerve-ring along the Uy / a rr Pi 
median line between y pp p 
estral form of bra d lids, 
anus, with atrophy -= CAP and aetna an eae = oe 
separation of the 
| ae YU CY) 
the two sides of the 
the mouthr and the Fie, 5 Diagram of Sedgwick’s hypothetical 
of the part behind opening; digestive tract and as i and nerve- 
