106 WILLIAM BATESON. 
Nearly all the proboscis cavity is eventually filled up with 
loose tissue. This is composed of a number of concentric rings 
of longitudinal fibres and connective tissue. These rings 
attain the maximum number of eight. In fig. 51 the general 
appearance of an older proboscis cavity is shown. The con- 
centric arrangement is, however, not yet attained in the stage 
there figured. 
A free space is always present between these rings of tissue 
and the central structures in preserved specimens. 
The proboscis gland becomes a large mass of tissue com- 
posed of anastomosing blood-vessels covered with conical cells 
fixed on the vessels by their apices. Many of these cells 
contain remarkable yellow granules, which are also to be 
found outside the cells, sometimes presenting a couglomerate 
arrangement. They would seem to be formed in the cells and 
thrown out. They are also to be found in the sac of the 
proboscis gland. This sac is blind posteriorly, but anteriorly 
the loose tissue which it contains passes into unbroken con- 
nection with the remarkable cellular layers covering the blood- 
vessels. Hence the sac is in communication with the central 
body cavity through the tissue spaces of the gland. The 
function of this gland is quite unknown. Spengel suggests 
that it is an “internal gill.” It does not seem probable to me 
that an animal with some sixty pairs of true branchial clefts 
would also possess another large and complicated organ of 
entirely different structure also for respiratory purposes. The 
presence of the brown granules suggests that it may be excre- 
tory. If this were so, the excreta might be expected to pass 
out by the proboscis pore which opens into the cavity in which 
the gland lies. No direct evidence was obtained as to the 
normal direction of the flow through this pore. Spengel and 
other observers state with regard to B. minutus that water 
is taken into the body cavity at the proboscis pore, but my 
own observations do not confirm this statement. On the 
contrary, particles of Indian ink or carmine held in suspension 
in the water in which the animals have lived for days, cannot 
be found to enter the proboscis cavity, while similar particles, 
