374 THE BIOLOGY OF MARINE ANIMALS 



(Fig. 9.3). In some varieties, such as the volvents, the thread is closed at the 

 ■ tip and forms a tightly coiled filament on discharge ; this filament wraps 

 itself around bristles and other projecting structures of the prey. In other 

 types the threads have open tips : these are discharged into prey with suffici- 

 ent force to penetrate the chitinous covering of small organisms. Often 

 the thread is armed with spiral rows of spines over some part of its length 

 and from the open tip a poison is discharged which paralyses the prey. 



Nematocysts are most abundant on the tentacles. They are plentiful 

 in the ectoderm of the oral region and become sparse towards the base of 

 the polyps. They also occur in profusion on internal structures, such as 

 gastric filaments, septal filaments and acontia (56). 



Cnidoblasts behave as independent effectors and are not subject to 

 nervous control. The responses both to normal stimuli and to localized 

 electrical stimulation are closely restricted to the affected region, and 

 repetitive stimulation is not followed by spread of excitation such as takes 

 place in certain regions of the coelenterate nerve net (Anemonia, Metrid- 

 ium). The effective stimulus for activating the cnidoblast is primarily 

 mechanical. Contact with solid food leads to a strong response, but 

 mechanical stimulation with inert solids has little effect unless the stimulus 

 is very strong (Fig. 9.4). Solutions of foodstuffs do not activate the cnido- 

 blasts but surface active agents such as saponin bring about a vigorous 

 discharge. Solutions of foodstuffs do permit the cnidoblasts to be readily 

 discharged by mechanical stimulation and it is concluded that the 

 foodstuffs sensitize the cnidoblasts chemically to mechanical stimulation. 

 The sensitizing substance is probably lipoidal in nature and adsorbed on 

 protein (92). 



Several theories have been proposed to account for the mechanism of 

 discharge. When the thread is everted the process begins at the base, which 

 is continuous with the wall of the capsule, and progresses towards the tip. 

 It has been suggested that increased pressure within the capsule forces out 

 the filament. If the capsule is acting as an osmometer, inflowing water would 

 build up the internal hydrostatic pressure when the nematocyst is activated. 

 An alternative theory supposes that the inflow of water leads to swelling of 

 colloidal material which causes eversion of the filament. In certain cnido- 

 blasts of Physalia contraction of fibrils may also be involved. It has been 

 shown that the uneverted filament in water undergoes anisometric swelling 

 and the process of discharge appears to take the following form. On 

 excitation water flows into the capsule, the internal pressure rises and the 

 operculum is forced off; the base of the filament, exposed to water, begins 

 to swell and moves outwards. As the tip advances a fresh region continues 

 to be exposed to water and hydrates, and the process proceeds until the 

 whole filament has everted. Meanwhile, the continued rise in intracapsular 

 pressure favours the forward movement of the tip and, by the turgor it 

 confers on the filament, assists it in penetrating prey (97, 107). 



Nematocyst poisons have been characterized to some extent, but their 

 exact nature has still to be determined. Some species are very virulent and 



