GASTEROSTEUS ACULEATUS. 75 



The shining 1 silvery pigment of the skin of the trunk is naked, except 

 where covered by the median dorsal shields and lateral armour that defends 

 the ventral side of the fish. 



The armature of the skin is sufficiently variable to almost suggest specific 

 differences, and on this evidence the species was divided by Cuvier into 

 Gasterosteus trachunis and (7. leinrus, or gymnurus. But when individuals 

 from many districts are compared, when the young- are compared with the old, 

 and those caught in summer are contrasted with fish taken in the winter, the 

 two extremes are found to be united by intermediate types, in which the 

 dermal shields show every possible gradation of form, as was pointed out 

 by the Swedish ichthyologists, Fries and Eckstrom. These naturalists re- 

 garded both colour and armature of the skin as varying- with summer and 

 winter, so that the Stickleback in its brig-htly-coloured bridal dress, when 

 it is naked, is Gasterosteus leiurus, while G. trachurus is the same species 

 in its winter dress. 



The form named Gasterosteus leiurus is .not met with except at the 

 spawning-time, though in the very young state, when only eight lines long, 

 examples of it have been described with slightly developed plates on the 

 pectoral and pelvic girdles. Dermal plates shield the back. In front of the 

 dorsal spine are two broad plates developed one behind the other, which are 

 in contact with the median shield of the occipital region; and shields of 

 varying size extend on each side, along the base of the dorsal fin, and often 

 reach the lateral shields. 



The lateral shields are best developed in the region of the pectoral girdle. 

 They are elongated vertically, in contact with each other, partly imbricated, 

 and vary in size and form, and in number from three to twenty-eight. 

 When most developed the bucklers descend to the abdomen, and have their 

 asperities finely furrowed. Their number is unequal on the two sides of the 

 body. There is a remarkable system of ventral armour which covers the 

 throat below the inferior angle of the operculum, and extends back to the 

 pelvis. The pelvis develops a shield which is sub-triangular behind, and 

 sends a branch up the side of the fish for about half the height of the 

 body; but it varies in form and ornament. This plate is the fulcrum for 

 the ventral fin. 



The course of the lateral canal along the upper third of the side of the 

 fish is very marked. The accessory gills are thin fimbricated filaments. The 

 ovary is double, but is stated by Fatio to be sometimes only deeply bi-lobed. 

 The air-bladder is simple, and ends in two short fine air-canals, which open 

 above the dorsal side of the stomach. The digestive canal is shorter than 

 the animal, and almost straight. 



