EFFECTOR MECHANISMS 401 



ism is unknown. Many marine animals also contain highly gelatinous 

 tissue which is very watery. This tends to reduce the density of the animal. 

 Various factors involved in counteracting gravity are described by Mar- 

 shall (85). Certain animals have floats or gas-bladders in which the gas 

 content is regulated and which act as hydrostatic organs. These can be 

 regarded as special effector organs. 



Gas-filled floats among invertebrates are found in siphonophores and 

 the cephalopod Spirula. The gas-float or pneumatophore of siphonophore 

 coelenterates such as Agalma lies at the top of the colony. In the Portu- 

 guese man-of-war Physalia and the by-the-wind sailor Velella the pneu- 

 matophores are capacious and the animals float at the surface, driven by 

 the winds that blow. In Stephanomia, for example, there is a gas-gland in 

 the floor of the pneumatophore, where gas is secreted, and a pore in the 

 roof through which gas can be released. This pore is guarded by a sphincter 

 muscle. If gas is removed from the pneumatophore it is refilled by secre- 

 tion within 30 min. By secretion of gas into the pneumatophore or release 

 through the pore the specific gravity of the colony can be regulated. 



Spirula is a small mid-water cephalopod which has an internal shell 

 containing a series of gas-filled chambers (p. 650). All these chambers are 

 completely enclosed except the last. The gas contained in the shell causes 

 the animal to float vertically with shell end uppermost. It is not known 

 whether the gaseous content, at least of the last open chamber, can be 

 regulated. 



The Teleostean Swim-bladder as a Hydrostatic Organ. The swim- 

 bladder of marine teleosts has several functions: it acts as a hydrostatic 

 organ, it is used for sound production and it aids in sound perception. As 

 a hydrostatic organ the swim-bladder brings the density of the fish to that 

 of sea water; consequently the fish is able to maintain position in mid-water 

 with minimal locomotory activity and expenditure of energy. The volume 

 of the swim-bladder of marine fish usually forms about 5 % of that of the 

 body, a value which gives a fish about the same density as sea water. Table 

 9.4 gives some values for the density of marine teleosts possessing swim- 

 bladders, together with the densities of some selachians and invertebrates 

 for comparison. 



A fish living in mid-water can be presumed to have its density adjusted 

 for life at the depth where it occurs. But when it swims up or down it is 

 subject to changes of hydrostatic pressure; these pressure changes alter 

 the volume of gas in the swim-bladder and change the density of the fish. 

 Thus, when a fish ascends, the gas in the bladder will expand, the fish will 

 become less dense and it will tend to rise to the surface. When it descends 

 the gas in the bladder will become compressed, the density of the fish will 

 decrease and it will tend to sink. To compensate for these changes in 

 density the fish can adjust the volume of the swim-bladder so as to bring 

 itself into hydrostatic equilibrium with its environment. 



The swim-bladder is usually closed in marine fish and its volume is 

 adjusted by addition or resorption of gas across its walls. When a fish 



