THE OEXl'S DIODON. 51 



with the belly upward. Xenopterus is a genus of the Indian Archipelago, distinguished by its funnel- 

 shaped nostril and the small dermal ossifications, which each have two or three roots and form spines 

 over the skin. The Xenopterus tnodestus, which is about four inches long, is found in the rivers of 

 Borneo and Sumatra, but the Xenopterus nariius has been met with indifferently in rivers and the 

 Sea of Penang. The specimens from Borneo reach a length of eleven inches. 



The large yellow Xenopterus naritus, which is found in the rivers of British Burmah, is con- 

 sidered to be excellent eating, but an allied genus (Tetrodon) from the Nile is known to be exceed- 

 ingly poisonous. Dr. Day records that a species from Japan (probably a Tetrodon) is used for 

 purposes of suicide, and that a law exists which prevents a soldier's son from entering the army when 

 his father has terminated his days by feeding on this animal. The Tetrodons are generally reputed 

 to be poisonous, but they are eaten by the Andaman Islanders, and the native doctors in Malabar 

 prescribe them as a medicine. 



The genus Tetrodon includes about sixty species, which are nearly all tropical, and by far the 

 larger number are known from the Malay Archipelago and adjacent seas, though well represented on 

 the African coasts. Several species inhabit fresh waters. In the first section of the genus the scutes 

 form a continuous carapace round the trunk ; the second group has a broad back and very 

 prominent nasal organs situate on elevated papillae, but there are no scutes forming a carapace. In 

 many species the spines are limited to the belly ; in others they occur on both the back and 

 belly, but are absent from the sides. Some have the spines minute and invisible, so that the fishes 

 are smooth to the touch ; others, again, have no spines at all, but minute soft tubercles on the 

 skin. A number of species have the back compressed into a keel, and in these the nasal organs are 

 never prominent. Tetrodon fluviatilis occurs indifferently in the fresh waters and on the coasts of the 

 East Indies. The little Tetrodon erytkrotcenia, two inches and a half long, is from the rivers of 

 Amboyna and Celebes. The entirely naked Tetrodon cutcutia, four inches long, is from the Ganges. 

 Tetrodon f aha ka, which is one of the largest species, reaching a length of eighteen inches, occurs all 

 up the Nile, in the Niger, and on the West Coast of Africa. The largest form, twenty-seven inches 

 long, is the widely- distributed T, sceleratus, which ranges round the Indian Ocean and through 

 Polynesia. 



The only British species, Tetrodon lagocepkalus, is also found on the south and east coast of 

 Africa, as far as Mauritius. The back is a brilliant ultramarine blue ; the belly and sides ai-e 

 silvery-white ; while the fins and tail are brown. The abdomen is covered with spines from the 

 mandible to the vent ; each has four roots. The back is almost straight. The air-sac has a smooth 

 internal surface, with two openings into the oesophagus, the first simple, and the hinder one valvular. 

 One of these is probably used only to admit the air, and the other to discharge it. It has been 

 found on the coasts of Cornwall and Ireland, the largest specimen being twenty-one inches long. 



The genus Diodon differs from Tetrodon in little beyond the absence of the median suture from 

 the jaws. The body is similarly covered with ossifications in the skin, each with a pair of lateral roots, 

 and a stiff movable and erectile spine. The upper part of the body is usually dark and the lower 

 parts white. The largest species (Diodon liystrix) is two feet to two feet six inches long. The four 

 species are found in all the seas between the tropics, and all range to the Cape of Good Hope. There 

 is no British representative of the genus. Mr. Charles Darwin remarks, in his " Voyage of the 

 Beagle" as quoted by Mr. Yarrell : " One day I was amused by watching the habits of a Diodon 

 which was caught swimming near the shore. This fish is well known to possess the singular power of 

 distending itself into a nearly spherical form. After having been taken out of the water for a short 

 time, and then again immersed in it, a considerable quantity both of air and water was absorbed by 

 the mouth, and perhaps likewise by the branchial apertures. This process is effected in two methods : 

 the air is swallowed, and is then forced into the cavity of the body, its return being prevented by 

 a muscular contraction which is externally visible ; but the water, I observed, entered in a stream 

 through the mouth, which was wide open and motionless ; this latter must, therefore, depend on suction. 

 The skin about the abdomen is much looser than that of the back, hence, during the inflation, the 

 lower surface becomes far more distended than the upper ; and the fish, in consequence, floats with its 

 back downwards. Cuvier doubts whether the Diodon in this position is able to swim : but not 

 only can it thus move forward in a straight line, but it can likewise turn round on either side. Tuis 



