RESPIRATION 167 



decline in frequency and amplitude of gill movements, resulting in com- 

 plete arrest after 30 min, with the gills closed. Excess C0 2 causes disorgan- 

 ization of the normal respiratory pattern, and in about 1 5 min leads to 

 complete stoppage with the gills gaping. The rapidity with which these 

 respiratory changes are effected has prompted the suggestion that extero- 

 ceptors are involved (155). 



Ascidians. In ascidians ciliary activity produces a more or less continuous 

 current over the gills, and this is augmented by rhythmical branchial and 

 atrial pumping or squirting movements (Fig. 5.17). Oxygen is taken up by 

 the vascularized test as well as by the branchial and atrial epithelia. Pump- 

 ing rates in Phallusia are estimated at about 225 c.c./hour, of which 60 c.c. 

 are due to ciliary through-current, the remainder resulting from contrac- 

 tions of the body wall. Oxygen utilization in solitary sea-squirts is low, 

 around 4-7% (17-20°C) (76, 81). 



Fishes. Breathing in fishes is regulated by a relatively autonomous 

 respiratory centre located in the medulla. In the skate this centre is capable 

 of continued activity when isolated from anterior and posterior levels of 

 the c.n.s., and possesses segmental functional regions corresponding to the 

 several gill arches which it controls. Respiratory movements are markedly 

 affected by peripheral stimuli, revealing well-developed reflex control. 

 Thus, decreasing the flow of water over the gills depresses respiratory and 

 cardiac rates and may lead to gasping movements {Raja, Scyliorhinus). The 

 circulatory centre is only weakly affected by changes in the blood circula- 

 tion; and when afferent pathways are anaesthetized by cocaine, respiratory 

 movements still continue at a reduced rate. It is believed that a double 

 mechanism for respiratory control exists in selachians, namely weakly- 

 developed automaticity of bulbar centres, on which are superposed strong 

 reflex reactions evoked principally by peripheral stimuli. Experiments 

 dealing with respiration have been carried out on teleosts, in which various 

 combinations of the IXth and Xth cranial nerves have been cut immedi- 

 ately medial to the gills. The results indicate that reflex activity, originating 

 in branchial receptors served by the IXth cranial nerves, is essential for 

 continued respiratory movements (126). 



Ventilation in fishes is very efficient and a large proportion of the oxygen 

 in the inspiratory current is withdrawn. Under normal conditions utiliza- 

 tion values for different species range from 46 % {Scyliorhinus, Spheroides) 

 to as much as 80% (Anguilla, Salmo, Uranoscopus). Fishes react to adverse 

 respiratory conditions (low 2 and high C0 2 tensions) in several ways. 

 Oxygen lack and heightened C0 2 result in more active swimming and 

 struggling in some fishes {Scyliorhinus, Anguilla, Opsanus), responses which 

 may have value in promoting escape from an adverse environment. Low 

 2 and high C0 2 tensions increase respiratory activity and ventilation. 

 The ventilation volume is altered by changes in depth, or frequency of 

 breathing, or both. In the eel ventilation may be increased fivefold when 

 the 2 content of the water falls below 4 c.c./l. In selachians {Mustelus, 

 Squalus) rhythmic respiratory movements show Q 10 values of from 1-2-2-4. 



