28 RESPIRATORY MECHANISMS 



again filled with sea water. There can be no doubt about 

 their respiratory function, since oxygen is absorbed from their 

 content, but they cannot be very effective. Bertolini (1935) 

 believed on the basis of inaccurate analyses that oxygen was 

 absorbed through their walls by a process of active secretion, 

 but v. Buddenbrock (1938) has shown that it is unnecessary to 

 assume anything but diffusion. When no mixing of the water 

 inside the "lungs" can take place the absorption by diffusion 

 must be a slow process. Although circulation of the body fluid 

 is described by Kawamoto (1927) it seems to be poorly de- 

 veloped and it appears possible that the alternate filling and 

 emptying of the respiratory tree may help to mix the contents 

 of the body cavity. 



In a number of Annelida, respiration through the walls of 

 the gut, acting as a lung by taking up water, has been as- 

 sumed, but the evidence is unsatisfactory. 



Eisig (1881) studied Hesione sicula, a rather robust worm of 

 a few cm length living in Posidonia {Potamogetonacece, nearly 

 related to ^ostera) meadows from the shore down to about 30 

 m along the coast of southern Italy. He found that the 

 stomach contains bubbles of gas, especially in two diverticula 

 which he calls swimming bladders. He assumes a regular 

 uptake of sea water and a separation of gas for respiratory 

 purposes from this water. He overlooked the fact that the 

 intercellular spaces of the plants regularly contain gas which 

 is also given off as bubbles, at least in the daytime. The 

 gas is mainly oxygen from the assimilation process, and it is 

 much more likely that the worms obtain their gas bubbles from 

 this source. It is quite possible that the oxygen obtained in 

 this way is of some respiratory significance, but there is no 

 proof, and cutaneous respiration may very likely be sufficient 

 for the animal's needs. Similar bladders, also containing gas, 

 were found in Syllis aurantiaca. 



Stephenson (1913) describes antiperistalsis and ciliary move- 

 ments ascending from the anus in a large number of Oli- 

 gochceta and Polychata, and assumes these movements to be 

 respiratory. Lindroth (1938) denies, however, that there is 



