30 CLASSIFICATION AND ADAPTATION 



body is more nearly rectangular than in other Flat- 

 fishes from the obtuseness of the snout and caudal 

 end, and the somewhat uniform breadth of the body. 

 The surface is rough from the presence of long 

 slender spines on the scales. There is a large 

 perforation in the septum between the gill cavities, 

 but this occurs also in Arnoglossus megastoma, 

 which is placed in another genus. But the generic 

 character of Zeugoptems, which is most important 

 for the present discussion, is the prolongation of 

 the dorsal and ventral fins on to the lower side of 

 the body at the base of the tail, the attachments 

 of these accessory portions being transverse to the 

 axis of the body. These fishes have the peculiar 

 habit of adhering to the vertical surfaces of the sides 

 of aquaria, even the smooth surfaces of slate or 

 glass. In nature they are taken occasionally on 

 gravelly or sandy ground, but probably live also 

 among rocks and adhere to them in the same way 

 as to vertical surfaces in captivity. Many years 

 ago (Journ. Mar. Biol. Assn., vol. iii. 1893-95) I 

 made a careful investigation of the means by which 

 these fishes were able to adhere to a smooth surface, 

 at least in the case of the largest and commonest 

 species Z. punctatus. It was observed that so long 

 as the fish was clinging to a vertical surface the 

 posterior parts of the marginal fins were in rhythmical 

 motion, undulations passing along them in succession 

 from before backwards, the edge of the body to 

 which they were attached moving with them. 

 The effect of these movements was to pump out 

 water backwards from the space between the body 

 and the surface it was clinging to, and to cause 

 water to flow into this space at the anterior edges 



