938 The American Naturalist. [November, 
7. ORIGIN OF VERTEBRATA. 
If the Annelids gave rise to the Chordata, we must consider 
the possible origin of certain anatomical characteristics of the 
Vertebrates, sensu strictu. Of such characteristics three are 
specially prominent, the general arrangement of the meso- 
derm, the pronephric duct, and the head with its arrangement 
of mouth, brain and eyes so unlike that in Annelids. 
1. The mesoderm of Vertebrates, as already remarked, is 
characterized by having a large splanchnocoele and by being 
segmented only in its dorsal or epiaxial part. In Amphioxus 
the mesoderm is at first totally segmented as in Annelids, and 
later develops a splanchnocoele, and assumes in other respects 
an arrangement (of sclerotome and cutis plate) wonderfully 
comparable, according to Hatschek, with the vertebrate type 
of mesoderm. Amphioxus seems to really afford the key for 
the transition of the mesoderm from the Annelid to the Ver- 
tebrate type, and the occurrence of the actual transition onto- 
genetically renders it natural to advance the theory of the cor- 
responding transition phylogenetically, thus obviating one of 
the former difficulties in the Annelidan theory of the origin 
of Vertebrates. 7 
2. The pronephrie duct, (often referred to as the Wolffian 
duct). This is a longitudinal epithelial tube, into which the 
segmental organs of Vertebrates open. Hatschek, in his paper 
on Polygordius, suggested that the segmental organs might 
acquire a direct secondary connection with one another; a 
series of such connections would make a longitudinal duct. 
Edward Meyer found such connections in adult Annelids, and 
other instances are known, see Willey’s Amphioxus, 80-82. 
Price’s observations on the Californian Myxinoid, and Rabl’s 
on Elasmobranchs, tend to show that this is the actual origin of 
the duct. We no longer, therefore, need to take refuge 1n 
Haddon’s or Boveri’s hypothesis of the origin of the duct, since 
Hatschek’s theory is sufficient and is supported by such actual 
evidence as we possess. i 
3. Cephalic homologies.—Up to this point we have dealt with 
knowledge and ideas derived from previous scientific publica- 
