2o A Guide to the Zoological Collections 



may contract and the dark cells expand to such an extent that the prevailing 

 colour of the fish may be dark brown ; but if, on the other hand, it be resting 

 on a hght surface, the dark cells may contract and the yellow cells expand 

 until the fish appears almost yellow. 



Or, as in the case of Batistes maculatus, the ordinary colouration is blue- 

 black with light blue spots, somewhat resembling that of certain unpleasant 

 sea-urchins of the reefs among which this fish is found. If, however, the 

 fish be resting against a very dark surface (as, for instance, a piece of' dis- 

 coloured drift-wood) the light-blue cells may contract until they are almost 

 invisible. 



One of the most « chameleonic » fishes of Indian Seas is Pterois miles 

 (Case 7) which, when creeping among the encrusted rocks of coral reefs, 

 presents almost every intermingled shade of red and brown, but which, when' 

 forced by pursuit into the blue water of the open lagoon, can become'almost 

 blue. 



In connexion with the integument it must be mentioned that in some fishes 

 its mucous secretion may be poisonous. In Indian waters there are many 

 such •' poisonous " fishes. The commonest are the Sting-rays, which by 

 means of a huge barbed spike on the dorsal surface of the base of the tail 

 can inflict a poisoned wound that may even prove fatal. 



Plotosus anguillaris is a Siluroid fish that with its large barbed dorsal 

 spine-and perhaps also with the similar spine of the pectoral fins-inflicts 

 a wound as painful as the sting of a hornet. 



In neither of these cases have any special poison-glands been discovered, 

 so that we must suppose that the secretion of the skin in the neighbourhood 

 of the spines is poisonous. In Synanceia verrucosa (Case 53 ), however, each 

 spine of the dorsal fin supports a cutaneous poison-bag, the spine' itself 

 being grooved to carry the poison into the puncture made by its sharp 

 point. F 



In connexion with the integument, too.it must be mentioned that some 

 fishes-especially among those species that either live habitually at ^reat 

 depths to which no sunlight penetrates, or that live at a considerable cTepth 

 of the open sea and only come to the surface at night-have luminous 

 organs imbedded in the skin. Specimens of several such species, that inhabit 

 the Indian Seas, are shown in the gallery. 



The Skeleton among Fishes occurs under two prin- 

 cipal types of form,— the BONY skeleton characteristic 

 of the large Order of Bony Fishes, and the CARTILAGIN- 



