158 FISH GALLEKY. 



Surgeon- The Surgeon-fishes, fishes belonging to the genus Acanthurus, 

 ts/^j are readily recognised by the single, sharp, lancet-shaped spine 



ease 15. with which each side of the tail is armed. When at rest the 

 spine is hidden in a sheath, when erected it is a most dangerous 

 weapon, which the fish uses by lashing the tail to right and 

 left (647, Acanthurus chirurgus, the Surgeon -fish proper; 650, 

 Acanthurus sohat, which is mounted with the back towards the 

 observer so that the spines may be the better distinguished ; and 

 651, a skeleton of Acanthurus /meatus, showing the relation of 

 the spine to the caudal vertebrae). The spines are absent in very 

 young individuals. The young Surgeon-fishes are more circular 

 in outline than the adults, and are more compressed and have no 

 scales ; they were formerly thought to be distinct fishes and were 

 described under the generic name Acronurus. 



The Acanthuridae are herbivorous or partly carnivorous fishes, 

 common in tropical seas in the neighbourhood of coral reefs; 

 they are fairly closely allied to the Chaetodontidae (Wall-case 14) 

 and lead on towards the File-fishes (Wall-case 20). The maxillary 

 is fused with the premaxillary bone, the mouth is small and not 

 protractile, the jaws have either bristle-like or incisor-like teeth ; 

 there are no teeth on the palate ; the post-temporal bone is fused 

 with the cranium. The scales are minute and rough, either 

 ctenoid or spiny; the dorsal and anal fins are long-based, the 

 dorsal has the spinous portion less developed than the soft portion 

 and not separated from it. Acanthurus lineatus (of which a 

 skeleton is shown, 651) is a poisonous fish, producing " Ciguatera" 

 similar to that caused by eating the flesh of the File-fishes. 



Unicorn- Naseus, the Unicorn-fish, 643-645, and fig. 71, has usually two 

 ^ a ' a ' spines at the side of the stalk of the tail, one in front of the other, 

 and these are not erectile as are those of Acanthurus. The 

 name Unicorn-fish is applied because of a bony horn projecting 

 forward above the mouth, sometimes two inches long in a fish of a 

 length of 20 inches. This horn is continuous with the bones of 

 the cranium (see skeleton 645), and is said to be used by the fish 

 as it butts up against the coral. Prionurus (642) differs from 

 Naseus in having a series of about six bony laminae on each side 

 of the tail instead of distinct spines. 



The Teuthididae (652-654) differ from the Acanthuridae in the 



