Respiration and Metoholisni 263 



groups. For further information concerning the ecological groupings and the 

 "microstratihcation" according to the axaiiability of o.wgen, Lindroth's paper--'' 

 should be consulted. 



Respiratory Control in Molluscs. Various kinds of breathing patterns arc 

 found in the ph\lum Mollusca by way of di\erse adaptations to oxygen pro- 

 curement. In the freshwater pulmonate gastropods, Lyiimaea, Helicosouia, 

 and PJjysa, the breathing rhythm, as e\'idenced by the duration of the inter\'al 

 between surfacings, is a function of oxygen need.''"' In water containing 

 approximately 6 cc. Oo/l. these molluscs remain submerged three times as 

 long as in water with only 2 cc. Oo 1. The temperature also influences sur- 

 facing interval, by its effect on activity and on dissolved oxygen. Indi\iduals 

 at 11° C. remain submerged 22 times as long as those at 21° C. 



The respiratory movements of air-breathing pulmonates have been in- 

 \'estigated, "■■■*• -•'^' '''^ and factors known to affect the \entilation rate are carbon 

 dioxide excess, oxygen lack, temperature, and humidity. •''*' Two respiratory 

 mechanisms are involved in the breathing regulation of this group— the control 

 of the pneumostome opening and the contraction of the muscle plate con- 

 tributing to the mantle lung. The stimulating effect of carbon dioxide in low 

 concentrations has been demonstrated in opening of the pneumostome in 

 Liviax, Helix, and Arion, and at a tension of 3-5 per cent COj the aperture 

 may remain open.^-^ The effect of oxygen decrease on pneumostome control 

 is questionable, but it is definitely effective in bringing about the contraction 

 of the sides of the mantle cavity, thereby increasing ventilation movements. 

 The role of hydrogen ion activity and of carbon dioxide increase have not 

 been elucidated in these pulmonates, although in the common oyster, Ostrea 

 virginica, the effect of acid, administered as HCl, is indicated by increased 

 water passage through the bivalve, subserving both ingestion and ventilation. 

 Oysters exposed for 6 hours to a pH of 6.75 to 7.0— somewhat below their 

 normal environmental range (pH 8.1)— continue pumping 95-98 per cent of 

 the time, considerably more than the normal rate. 



The cephalopods have a highly developed state of respiratory control with 

 a nerve action analogous to that of the Hering-Breuer reflex of higher verte- 

 brates. Winterstein studied the effects of respiratory stimulants on the ventila- 

 tion of Octopus and showed that carbon dioxide is a very potent factor in 

 increasing the frequency and, to a lesser degree, the amplitude of breathing 

 movements.'"^ The volume of water pumped through the mantle cavity of one 

 experimental animal increased from 230 to 1075 cc. per minute under the 

 influence of carbon dioxide. Winterstein emphasized the importance of hvdro- 

 gen ion activity rather than COo as the effective respiratory stimulant.'"^ 



Breathing Movements in Crustacea. Although body motion, abdominal 

 flexion, and leg movement are important for ventilation in Crustacea, the two 

 respiratory mechanisms which have been most thoroughly studied experi- 

 mentally are scaphognathite beat and pleopod movement. Increase in tempera- 

 ture causes an increase in rate and amplitude of scaphognathite beat.^'*'' As 

 to the stimulating effect of oxygen deficiency on rate of ventilation there is 

 general affirmative agreement among investigators, most of whom have dealt 

 with the European crayfish, Astacus fluviatilis. However, there is consider- 

 able disagreement regarding the effects of carbon dioxide on this species. ^'^• 

 197, .314, .323 Some of the difficulty in determining the precise effect of carbon 



