THE TRIGGER-FISH 



surrounded by a thick calcareous skeleton which acts 

 both as a support and as an envelope. Beside the 

 worms with naked teguments we find others which 

 shelter themselves in a mucous or calcareous tube 

 which they have themselves produced. The vast 

 Mollusc group is characterized by the frequent 

 possession of a calcareous shell, 

 either single or divided into 

 two valves; in this class crea- 

 tures without any protection 

 or supporting structure are in 

 the minority. Then there is 

 the still more extensive group 

 of articulated animals, includ- 

 ing the Crustacea and Insects, 

 which have an external skele- 

 ton, sometimes confined to a 

 sheath of chitin, a horny or- 

 ganic substance produced by 

 the skin, sometimes thickened 

 into a carapace, an external 

 cuirass, the chitin of which is 

 strengthened by lime. So we 

 come to fish and other verte- 

 brates, and see that the pos- 

 session of surface armour, and 

 internal skeletal constructions, 

 is not a new or special attri- 

 bute at all, but only the sign of a general condition. 



The trigger-fish and the mailed gurnard are not the 

 only armoured fish in Nature today. There are 

 others, some of which we have already mentioned, 

 such as the sea-horse with its grasping tail, the coffer- 

 fish with its clumsy body, the bichir and bony pike 

 which live in the fresh waters of Africa and America 

 respectively. In the last of these, the cuirass extends 

 all round the body and is made up of plates, the main 

 substance of which is bony. This protects the inside 

 82 



Fig. 15. — Fossil armoured fish 

 (A sterolepis cornutus) found 

 in Devonian strata of the 

 Palaeozoic era. 



