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. 



