442 - WILSON. (VoL. III. 
transverse axes of the gastrula, and hence of the Ccelenterate 
ancestral type, which the gastrula in some degree represents ; 
and we owe to Balfour the fruitful suggestion that the nervous 
system of the Bilateralia may have arisen directly by the elonga- 
tion of the circum-oral ring of the ancestral form. Kleinenberg 
has shown in the clearest manner that this view is untenable, if, 
as Balfour supposed, and Kleinenberg himself believes, the Tro- 
chosphere is an ancestral form; for the adult nervous system 
does not arise from the ring-nerve of the larva, and lies at right 
angles to it. This objection, however, rests wholly on the sup- 
posed homology of the Trochosphere ring-nerve with the circum- 
oral ring-nerve of a medusoid form; and this homology, to say 
the least, remains to be proven. On the other hand, the adult 
nervous system, like the larval “ring-nerve,” certainly surrounds 
the region of the blastopore, as I have endeavored to show ; and 
if there is any force in the foregoing argument, the larval ‘“‘ring- 
nerve” is essentially a secondary system, developed in connec- 
tion with a purely larval locomotor organ. The possibility of 
such an origin is rendered apparent by the fact that Polygordius, 
and presumably other larval types as well (see Fraipont, No. 15), 
has not one but several (in Polygordius six) parallel ganglion- 
ated nerve-rings, of which only two supply the locomotor organ, 
while the others form part of the general umbrellar nervous 
system} 
Grant that the ring-nerve of the Trochosphere is essentially 
a secondary larval structure, and the objections to Balfour's 
fundamental conception disappear, though some of the details 
of his hypothesis must be modified. 
XIV. MESOBLAST AND C@Lom. 
Our knowledge of the mesoblast in annelids appears at pres- 
ent to be in a very confused and unsatisfactory condition. In 
all the foetal types (Oligochzeta, Hirudinea), and in many of the 
1 Fraipont (/c., p. 56) is inclined to regard the fifth nerve-ring of Polygordius 
(which supplies the prae-oral ciliated band, and which alone is provided with 
ganglion-cells throughout its entire course) as the representative of the ring-nerve 
of Lopadorhynchus and similar forms. Whether this view be well founded or 
not, the prototrochal nervous apparatus of Polygordius differs remarkably from 
that of Lopadorhynchus, etc., both in position and in structure. 
