8 THE FRESH-WATER FISHES <)E EUROPE. 



The gill-cover consists typically of four thin bony plates. First, there is an 

 expanded hindermost plate, which is named the operculum ; secondly, u long- 

 plate beneath the operculum, named the sub-operculum ; thirdly, the inter- 

 operculum, which prolongs the line of the sub-operculum forward ; and, 

 fourth, the pre-operculum, which is elongated vertically, and extends in front 

 of the other opercular elements. The gill-cover is united with the suspensory 

 arch, which connects the lower jaw with the skull, the opercular bone being 

 united to the hyo-mandibular bone, and the inter-operculum has a ligamentous 

 union with the suspensory arch, as well as with the lower jaw. A modified 

 gill, which possessed respiratory functions in the embryo, but has lost those 

 functions in the adult, is attached to the operculum. These gills are known 



Fig. 4. SKELETON OF PEKCH. 



a, Pre-maxillary bone; I, maxillary bone; c, mandible; d, palatine arch; e, cranium;/, iuter-operculiaui ; 

 0, !/') vertebral column ; h, pectoral fin ; i, ventral fin; fc, I, dorsal fins ; m, anal fin; n, n', caudal flu. 



as pseudobranchise. They do not exist in all fresh-water fishes, and when 

 present, carry aerated blood to the eye and other parts of the head. The 

 gills are supported by the branchial arches ; most bony fishes have four 

 complete gills. The branchial arches commonly develop protuberances, which 

 are called gill-rakers, and form a sort of net to filter the water from solid 

 substances, which might otherwise be carrried into the gill-cavity. The 

 number of branch i-ostegal rays, and the degree of development of the 

 gill-rakers on the branchial arches furnish important characters in defining 

 the species of fish. In the Sturgeon there is an external opening to a canal 

 placed behind the orbit, which is known as a spiracle. These spiracles lead 

 into the pharynx. In Lampreys there are a number of successive respira- 

 tory sacs, each opening externally by a short duct, and internally into a canal, 

 which communicates in front with the back of the throat, termed the pharynx. 

 In most fishes the water is taken in through the mouth, and passed out by 

 the gill-openings. 



