SKELETONS, SHELTERS AND SPECIAL DEFENCES 665 



a sticky slime which swells to enormous extent on contact with sea water. 

 When irritated Holothuria contracts strongly, rupturing the cloaca, and 

 ejects its viscera, including the Cuvierian organs. These elongate greatly 

 when autotomized, seemingly as the consequence of raised hydrostatic 

 pressure, and unfolding of spirally wound fibrils. When an enemy comes 

 into contact with these sticky tubes, they become pulled out into a snarl of 

 glutinous white threads, in which the predator may be completely en- 

 tangled and immobilized. Cuvierian organs sometimes contain a highly 

 toxic material termed holothurin. Extracts from Actinopyga are lethal to 

 fish and mice, and poison protozoan cultures (24, 33a, 41, 6\a, 91, 92). 



Fishes with Poisonous Flesh. Certain fishes are known to have poisonous 

 flesh and to cause food-poisoning when eaten by man. Poisons are localized 

 in particular organs, e.g. gonads, or are widespread throughout the body 

 of the fish. There are instances of fishes, such as square-tails, wrasses and 

 parrot-fishes (Tetragonuridae, Labridae and Scaridae), which occasionally 

 become toxic through having eaten poisonous food. In the file-fishes 

 (Monacanthidae) and trigger-fishes (Balistidae) the flesh is reported to be 

 poisonous, at least at certain seasons. Still other fish are more or less 

 permeated with poison at all times. Puffers and globe-fishes (Tetraodonti- 

 dae) are always more or less poisonous to man. 



The poisonous principles are classified as endotoxins or ichthyosarco- 

 toxins, but their chemical nature is unknown. The relative toxicities of 

 various tissues from puffer-fish have been determined by tests on labora- 

 tory animals. Liver shows high incidence of toxicity ; frequently gonads, 

 intestine and muscles are poisonous as well. The principal symptoms are 

 neurological disturbances and gastric derangements. Information is not 

 available to answer the natural query whether the poisonous character of 

 the flesh of these fishes has survival value against predators other than man 

 (43, 50,54,93, 122, 133). 



Venomous Stings and Spines among Fishes. Some fishes with spines for 

 stinging or lacerating have special glands for producing poisonous secre- 

 tions in conjunction with these devices. Like the poisonous fangs of snakes 

 these devices in fishes probably have offensive as well as protective func- 

 tions. An interesting account of poisonous fishes is available in Evans' 

 Sting-fish and Seafarer (34). 



Poisonous spines occur in several elasmobranchs, the most formidable 

 of which are sting- and eagle-rays (Dasyatidae and Myliobatidae). In 

 these animals there is a large serrated spine on the tail which is capable of 

 inflicting severe wounds when the tail is lashed from side to side (Fig. 

 15.17). Along either side of the spine there is a narrow groove containing 

 specialized glandular tissue which produces a powerful venom. Some of 

 this material becomes injected into any wounds inflicted by the spine. The 

 poison of the sting-ray causes great pain, paralysis and swelling of the 

 affected part in man. The poisonous spine of Trygon {Dasyatis) is not an 

 effective protection against the natural foes of this fish, namely hammer- 

 headed and tiger sharks. A specimen of the former is reported to have had 



