109 



change in direction of motion to one side could be effected by 

 the sudden erection of the fin on that side while the other was 

 kept close to the body. A sudden change in direction is of 

 course in ordinary fishes more usually brought about by a 

 flexure of the tail or the whole body. The probable meaning 

 of the habitual use of this peculiar method of steering was made 

 apparent when the fish was swimming in circles, which it often 

 did. The tail was then used only for propulsion, while the 

 ventral fin on the one side kept the course to a circle, being 

 held continuously erected. This habit may be connected with 

 its peculiar mode of life in association with the shark, for there 

 is some danger of its being left behind, should it wander too 

 far off. 



These fish often attach themselves to ships and large fish, 

 like sharks and whales, and sometimes to turtles. Natives on 

 the east coast of Africa, as is well known, catch the last-named 

 animal by the curious fishing tackle of a live Remora on a 

 string. The fish will fasten on and keep its hold with the 

 unreasoning obstinacy of this peculiar instinct 



The Blaasop {Tetrodon honckenii, Bl.). 



The general behaviour of this fish differs markedly from 

 that of any other observed ; in this respect and in its rather 

 bizarre appearance, it offers a striking contrast to the other 

 inhabitants of the tank. It usually swims about quite un- 

 concernedly, apparently without fear of attack. Fish usually 

 owe their power of locomotion to the use of the powerful 

 muscles of the tail, and by a single powerful stroke are able to 

 project the body through the water at a very rapid rate. Even 

 fish which usually move comparatively slowly can in this way 

 escape danger. The Blaasop, however, except on rare occa- 

 sions, was observed to use only its fins, the dorsal, ventral and 

 pectoral for locomotion, in a sort of sculling or paddHng 

 manner. The fish can therefore by these same means preserve 

 its balance, and the absence of pelvic fins, which are generally 

 used for this purpose in other fishes may thus be accounted 

 for. The muscles of the tail are not well developed, and this 

 organ with the rather elongate caudal fin is used as a steering 

 organ, seldom as a locomotory organ. When it occasionally 

 hurries along at an increased speed for food, the fins used in 

 progression merely flicker to and fro more rapidly. It rarely 

 darts about, but proceeds at a more or less uniform rate in a 

 deliberate manner. Nor is it protected by colour, being, on 

 the contrary, rendered rather conspicuous by large yellow 

 spots scattered over the surface of the body. The head is large 



[C.P. 6-'i4.] 



