INTRODUCTION TO THE FISHES 205 



as the modern representatives of the earliest type of fish, as so 

 far demonstrated by fossil remains. 



Though the external form of fishes is subject to great variety, 

 being sometimes spherical, or cubical, or expanded, or snake-like, 

 it is generally that of an elongated oval, slightly compressed later- 

 ally, a shape which enables the fishes to traverse the water with 

 the greatest celerity and ease. Every part of the body is most 

 wonderfully adapted to its aqueous environment ; the fins, tail, 

 and the motion of the whole backbone assist progression, and 

 produce that astonishing ease and rapidity of movement which is 

 so characteristic of fishes. Fishes possess paired front and hind 

 limbs, called the pectoral and pelvic fins, which, although not 

 resembling in any detail the limbs of any land animal, are certainly 

 the forerunners of the arms and legs of terrestrial vertebrates. 

 They are flattened structures, subject to considerable modifica- 

 tion of form and size, to which are attached numerous rays serving 

 to stiffen the membrane of the fin. The other fins on the back 

 and belly of the fish are single or unpaired, and are entirely unre- 

 presented in the skeletons of higher land vertebrates. 



The modifications of the gills are sufficiently important to 

 give the names Marsupibranchii, Lophobranchii, and Elasmo- 

 branchii to certain groups of fishes. In the hag-fish, which is an 

 example of the marsupial type of gill, there are six little branchial 

 sacs on each side ; these are produced into short tubes on both 

 sides, and these tubes are prolonged into a longitudinal canal, 

 which extends backward, and carries the stream of water away 

 from the gills on each side, terminating on the ventral surface on 

 each side of a third larger opening, which admits water in the same 

 way into the branchial sacs. In the pipe-fishes (Lophobranchii) 

 the gills form a double series of nearly circular tufts, instead of the 

 comb-like form usual in fishes. In the sharks, dog-fishes, and 

 rays (Elasmobranchii) the gills are very numerous, and there 

 may be as many as seven gill-pouches, opening separately by slits 

 to the exterior, the first gill-cleft being modified to form a special 

 tube, called the spiracle, and only carrying rudimentary gills. In 

 the other fishes this spiracular gill-cleft does not open to the ex- 

 terior, and is quite rudimentary, while in the land vertebrates we 

 find it converted into the cavity of the middle ear, and its opening 

 into the throat remains as the Eustachian passage. The branchial 



