MOLGULA 139 
It is not a part of the body-cavity, but has been formed by an 
infolding of the outer surface of the body. Into it, near the 
base of the excurrent siphon, the digestive and genital tracts 
discharge their products for removal with the current of 
respiratory water which streams out of that siphon. 
The respiratory system. The principal respiratory organ is the 
pharynx, which communicates with the incurrent siphon by 
an opening fringed with a circular row of branched tentacles. 
Its walls are pierced by numerous slit-like, ciliated openings, 
called stigmata, through which the respiratory water streams 
from it into the peribranchial chamber. A current of water is 
thus maintained, which passes through the incurrent siphon 
into the pharynx, and thence through the stigmata into the 
peribranchial chamber, and out again at the excurrent siphon. 
The stigmata are vertical in position and are arranged in trans- 
verse rows, which extend across the pharyngeal wall, and are 
separated from one another by delicate vertical bars; the trans- 
verse rows have between them large transverse bars, and running 
longitudinally along the pharyngeal wall on each side are six 
large longitudinal bars or ridges, which are easily seen and have 
already been mentioned. Through all of these bars the blood 
circulates, being brought to them either by the cardio-branchial 
or the viscero-branchial blood vessels, and respiration is thus 
carried on. 
Lay the animal with the left side uppermost. Slit open the 
incurrent siphon and the pharynx by inserting the point of fine 
scissors into the siphon and, after cutting its wall to its base, 
carrying the cut through the wall of the pharynx along the side 
of and parallel with the mid-ventral line to the posterior end of 
that organ. Lay the pharynx open. The twelve large longi- 
tudinal bars will be seen projecting into the pharyngeal lumen. 
Trace them throughout their entire extent. Find with the aid 
of a dissecting microscope or a hand lens the row of branched 
tentacles at the base of the incurrent siphon and count them. 
