378 Report of the Microscopical Section. [Sess. 
Cutting open the body wall, we come to a large organ called 
the branchial sac. On the ventral side the branchial sac is 
contiguous with the body wall, but on every other side there is 
a considerable vacant space between these two. The part of 
this vacant space at the dorsal side of the branchial sac and 
extending to the atrial aperture is called the atrial cavity, 
while the vacant space on the other two sides is called the 
peribranchial cavity. Extending through this peribranchial 
cavity are numerous connectives which bind the branchial 
sac to the body wall. 
The wall of the branchial sac deserves close examination. 
It consists of three distinct series of bars or blood channels :— 
1. The transverse vessels which run horizontally round the 
wall and open into two large vessels which run longitudinally 
—the one on the ventral side called the ventral blood sinus, 
and the other on the dorsal side called the dorsal blood sinus. 
2. The five longitudinal vessels which run vertically between 
the transverse vessels and open into them. Between these 
longitudinal vessels there are five slits called stigmata. 
3. The third series of vessels, called the “internal longi- 
tudinal bars,” run vertically in a plane internal to the previous 
two series, but are attached to the transverse vessels by short 
tubes at the crossings, and at these points short papillz project 
into the branchial sac. 
The ventral and dorsal blood sinuses communicate with 
each other by means of a transverse vessel near the branchial 
aperture, so that there is thus intercommunication throughout 
the whole of the wall of the branchial sac. 
Proceeding with our dissection, we cut open the wall of the 
branchial sac right through the branchial aperture, thus laying 
this latter out flat. We then find that at the margin of the 
branchial aperture there is a slight infolding of the test. A 
similar infolding takes place at the atrial aperture. 
Round the neck of the branchial aperture, and also round 
that of the atrial aperture, run strong muscles called the 
sphincter muscles: these sphincter muscles resemble much 
in appearance and function the india-rubber bands which 
formerly used to be attached to the wrist part of some 
cloth gloves. 
Below the sphincter muscle at the branchial aperture is a 
