HABITS. 211 



Teleosteans frequently undergo remarkable changes of form 

 in their growth. This is a marked feature of the group, and 

 leads to some difficulty in the recognition of species. As ex- 

 amples may be mentioned the Pleuronectidae, Cyttidae, 

 Muraenidae, Xiphiidae, Plectognathi. In many cases the young 

 are so different that they have been described as distinct genera. 



Moreover, Teleostean fishes are often highly variable under 

 the influence of changed conditions (variation in acquired char- 

 acters), particularly with regard to colour, both of skin and flesh. 

 The change in the colour of the skin is due to the pigment cells 

 ( chroma tophors). 



Secondary sexual differences are usually present. The male 

 is generally smaller than the female, and some of its fin-rays or 

 fins may be prolonged. The male is often more brightly coloured 

 in the breeding season, or its skin may become warty. 



Hybridism is also known to occur (Serramcs, Pleuronectidae, 

 Cyprinidae, Salmonidae, etc.). 



Some fishes are very long-lived (carp and pike to beyond 100 

 years), and growth frequently appears to be somewhat indefinite 

 and to continue for a long time. Fishes which rapidly a,ttain to 

 their full size (e.g. sticklebacks) are said to be shortlived. 



A few fishes have been domesticated and transported to 

 different parts of the globe (carp, Crucian carp, tench, goramy), 

 and certain species of salmon and trout have been acclimatised 

 in countries in which they are not indigenous (see accounts of 

 families). 



Marine fishes are usually extremely sensitive to changes of 

 temperature, freshwater fishes much less so. It is said that the 

 carp will survive after being frozen in a block of ice. A modified 

 hibernation has been observed in some Cyprinoids and Muraen- 

 oids in cold weather, and many tropical fishes (Siluroids, 

 Labyrinthici, Ophiocephaloids, etc.) pass the dry season in a 

 torpid state in hardened mud. 



The flesh of many fishes is poisonous, and in unknown waters, 

 especially in tropical seas, great care must be exercised in select- 

 ing fish for food. 



The wounds caused by the spines of many fishes are poisonous. 

 This is generally due to the poisonous nature of the mucus which 

 covers the body, but it may be caused by special poison glands, 

 as in Synanceia, Thalassophryne. 



