212 THE ORIGIN OF VERTEBRATES 
original opercular ducts by which the generative products were con- 
veyed to the uterine chamber, z.c. to the chamber of the thyroid 
gland, and thence to the common genital and respiratory cavity, and 
so to the exterior. 
It is easy to picture the sequence of events. First, the generative 
glands, chiefly confined to the cephalic region, communicating with 
the exterior by separate ducts on the inner surface of the operculum 
asin Limulus. Then, in connection with the viviparous habit, these 
two oviducts fused together to form a single chamber, covered by the 
operculum, which opened out to the exterior by a single opening as 
in Scorpio: or, in forms such as Eurypterus, in which the operculuin 
had amalgamated with the first branchial appendage and possessed a 
long, tongue-like ventral projection, the amalgamated ducts formed 
a long uterine chamber which opened internally into the genital 
chamber—a chamber which, as in Thelyphonus, was common with 
that of the two gill-chambers, while at the same time the genital ducts 
from the cephalic generative material opened into two uterine horns 
which arose from the anterior part of the uterus, as in Thelyphonus. 
Such an arrangement would lead directly to the condition found 
in Ammoceetes, if the generative material around the brain lost its 
function, owing to a new exit for generative products being formed 
in the posterior part of the body. The connection of the genital duct 
with this cephalic gland being then closed and cut off by the brain- 
case, the position of the oviducts would still be shown by the ciliated 
grooves opening into the folded-down thyroid tube, i.e. the folded- 
down horns of the uterus; the uterus itself would remain as the 
main body of the thyroid and still open by a conspicuous orifice into 
the common respiratory chamber. Next, in the degeneration process, 
we may suppose that not only the oviducts opened out to form the 
ciliated groove, but that the uterine chamber itself also opened out, 
and thus formed the endostyle of Amphioxus and of the Tunicata. 
It might seem at first sight improbable that a closed tube should 
become an open groove, although the reverse phenomenon is common 
enough; the difficulty, however, is clearly not considered great, for it 
is precisely what Dohrn imagines to have taken place in the conversion 
of the thyroid of Ammoccetes into the endostyle of Amphioxus and 
the Tunicata; it is only carrying on the same idea a stage further to 
see in the open, ciliated groove of Ammoccetes the remains of the 
closed genital duct of Limulus and its allies. 
