671 
which are the extremities of the blastopore is a part of the body formed 
by the closure of the blastopore lips. 
And now but little is required to convert this embryo into a typi- 
cal trochosphere larva. The digestive tract becomes hollow, and ac- 
quires two openings to the exterior at the two points of its previous 
connection with the ectoderm. The one nearer to the ciliated band 
becomes the mouth, while the other becomes the anus. The region 
between these two openings, formed as we have seen by the closed 
blastopore lips, becomes the ventral surface. The ciliated band origi- 
nally around the blastopore becomes the preoral band of the trocho- 
sphere, showing that the body of the trochosphere (the postoral lobe), 
is formed, as above described, by the elongation of that part of the 
gastrula situated within this circumblastoporal band. According to 
this description it will be seen that the blastopore does not form the 
mouth nor anus exclusively. It elongates and closes but the mouth 
subsequently appears at one of its extremities and the anus at the other, 
while the intervening portion closing becomes the ventral surface of 
the animal. 
This is a point of considerable importance. It will be noticed that 
the history of the blastopore in Serpula, is very similar to that of Per 
patus as described by Balfour, and it is also strictly in accordance 
with the view of Sedgwick as to relation of the blastopore to the 
future digestive system. It is certainly an important corroboration of 
this view of Sedgwick’s, that Annelids which show other evidences 
of a primitive development should so closely agree with the theory. 
Just before the formation of the anus, a number of endodermal 
cells near the region of the future anus become separated from the rest 
of the digestive tract and form a mass of cells lying outside the alimen- 
tary canal in the body cavity. These cells form the mesoderm. Some 
of these cells increase in size and form stellate mesenchyme cells, and 
finally a few of them stretch across the body cavity near the anus form- 
ing a membrane which separates a small portion of the body cavity 
from the rest. This small portion at the extreme posterior, is the anal 
vesicle described by Stossich. It has no connection with the alimen- 
tary canal. It is always a very characteristic feature of the Serpula 
trochosphere and will readily distinguish it from others. Occasionally 
another partition grows across it separating it into two smaller divi- 
sions as was observed by Stossich. 
Certain others of the above mentioned mesoderm cells have a 
different history and form the true mesoderm. They multiply quite 
rapidly and soon give rise to two bands of cells one on either side of 
the alimentary canal and extending forward toward the mouth. They 
