FISHES. 263 



the present justifies the separation of the forms for which it was proposed, from that 

 sub-order. 



The PoLTNEMiD^, the only family of Rhegnopteri, are fishes of a more or less 

 sub-fusiform shape, although sometimes compressed, with normal ctenoid scales, and 

 with the head ending in a rather blunt, although protuberant snout ; the mouth is 

 well cleft and sub-inferior ; there are two dorsals widely separated ; the first with 

 seven or eight spines ; the last, as well as the anal, oblong ; the inferior rays of the 

 pectoral fin are entirely separated from each other, and capable of independent move- 

 ments, all being flexible forwards ; they vary in number from three to fourteen. The 

 species are inhabitants of the tropical seas, and several of those on the Indian coast 

 are of considerable economical importance. The air bladder is extremely diverse, in 

 some it being very largely developed and with appendages, while in other species, 

 very little differing from them in appearance, it is very small or entirely wanting. 

 The large air bladders form a considerable part of the isinglass of India. The largest 

 of the family reach a length of five or six feet, but most species are very much smaller, 

 and the American species are little more than a foot in length, or even less. 



Three species visit or occur along the eastern American coast, the Polynemus 

 plumieri, P. octojilis, and I^. octonemus. 



SxJB-OeDEE XI. — DiSCOCEPHALI. 



Another sub-order has been constituted for fishes distinguished by the develop- 

 ment of a suctorial disk on the upper surface of the head. Only one family — the 

 Echeneididae — is known. 



The EcheneididjE are of great interest, and peculiar in the possession of a sucker-like 

 attachment to the upper surface of the head, which has procured for them the popular 

 name of sucker, suck-fish, and sucking-fish. These have a body quite elongate, wide 

 at the front, and gradually tapering backwards ; the head above is very flat, and on a 

 line with the back, and below regularly curved upwards to the margin ; the eyes are 

 high up and over-arched by the sucker ; the mouth has a moderate oblique cleft ; the 

 dorsal fins are two, but the first is so extraordinarily modified, that, to use a Hiber- 

 nicism, it is not a fin at all ; the sucker that surmounts the head is itself nothing 

 but a greatly modified first dorsal whose spines have become cleft, the membranes 

 thickened and leathery, and all subjected to accompanying modifications which result 

 in the remarkable sucker ; the true dorsal and the second anal are far behind and of 

 moderate length ; the pectorals are high up, and their upper edges near the back ; 

 the ventral fins are thoracic and composed of a slender spine imbedded in the skin, 

 and five rays. 



The aspect of the fish is so peculiar that an inexperienced observer is very apt to 

 mistake, and, in fact, generally does mistake, the back for the belly and vice versa. A 

 glance at our illustration will readily show this. 



The species of the family to some extent are parasitic, attaching themselves by means 

 of their suckers to large fishes, especially sharks and sword-fishes, and being in this 

 way borne about, detaching themselves occasionally to obtain food. The tenacity 

 with which they adhere to objects is such that they have been used by some peoples 

 to catch turtles and other fishes. 



One of the oldest accounts of their use for fishing was given by Columbus or one 



