20 



SORTATION. 



of all these stages are still found, and not only so, but in many 

 cases one or other of the fins is rudimentary or absent, and in 

 some they form an adhesive disk or sucker. 



The caudal fin is the fish's main propeller, with the paired fins 

 he retains his trim, and the dorsal and anal he uses as a keel 



to keep him straight on his 

 course. With his tail he docs 

 most of his steering, but he 

 brings in his pectorals to help 

 him. A stroke of the tail to 

 starboard and a stroke or two 

 with the starboard pectoral, 

 with the port pectoral held 

 against the body, sends his 

 head to port ; a stroke of the 

 tail to port with a stroke or 

 two of the port pectoral, and 

 the starboard pectoral held 

 in, sends him round to star- 

 board; in fact, the pectoral 

 thrown out of action serves 

 as a pivot on which he turrs. 

 Cut off a pectoral and he falls 

 on the opposite side ; cut off 

 both and his head sinks ; cut 

 off all his fins and he capsizes, 

 owing to his upper half beii g 

 the heavier. 



On the other page we have 

 (Fig. i) an outline sketch of 

 the body of a perch with the 

 backbone removed to show 

 more clearly the fins and 

 their attachments ; and here 

 (Fig. 2) is an outline of one 

 of the perch family Dentex 

 showing the fins and the 

 " lateral line " we shall meet 

 with immediately. It will be 

 noticed that the perch has two 

 dorsals and the dentex only 

 one, though the one has a 

 depression between the spines 

 and rays where the division 

 is beginning. A fin may be 

 defined as an expansion of the integument supported by radiating 

 rods; when the rods are bony they are called "spines"; when 

 they are cartilaginous they are known as " rays." A spine is 

 always fairly stout and sharply pointed ; a ray is generally soft, 

 and may be simple or articulated or branched, the branching 

 taking the form shown in Fig. 3. 



Enough of this for the present. Let us return to our line of 

 enquiry with the group in which the tail has a longitudinal fold. 



Fig. 2. LATERAL LINE (Dentex). 



