SURVEY OF INVERTEBRATES 87 



pendent on the tension (Tang, 1931; Tang and Gerard, 

 1932) is of interest, for it suggests the possibility that 

 part of the required energy may be derived from anaero- 

 bic processes at low tensions. 



A curious observation, the significance of which is not 

 yet fully apparent, is that anaerobiosis definitely de- 

 creases the permeability of the Arbacia egg to water; it 

 has no influence, however, on the equilibrium volume of 

 the eggs when the latter are exposed to 66 or 80 per cent 

 sea water (Kekwick and Harvey, 1934). 



Another remarkable fact is that the prevention of oxy- 

 gen consumption, either by the use of cyanide or by im- 

 mersion in oxygen-free sea water, increases by several 

 hours the period during which unfertilized starfish eggs 

 may be fertilized by sperm, or activated artificially. The 

 period necessary to induce activation by heat or acid also 

 becomes shorter with increasing length of the anoxic 

 conditions (Lillie, 1931). 



V. MOLLUSCS 



Among the molluscs, the anaerobic functions are most 

 pronounced in the lamellibranchs, they are little devel- 

 oped in gastropods and seem practically non-existent in 

 cephalopods. These differences are clearly related to 

 ecological conditions as will become apparent from the 

 following discussion. 



1. LAMELLIBRANCHS 



A. Occurrence in anaerobic habitats and tolerance of 

 experimental anaerobic conditions. Lamellibranchs, in 

 general, show little locomotion; many are even truly 

 sessile organisms. Consequently they cannot readily es- 

 cape asphyxiation by wandering away if the oxygen dis- 

 appears in their surroundings. Many clams live buried 

 in the mud, where, as shown previously, severe oxygen 



