8 THK KKESII-WATEK FISHKS OF KT'HOPK. 



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

 expanded hindermost plate^ which is named the operculum ; secondly, a lono;' 

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

 operculum, which proloni^s the line of the sub-operculum forward ; and, 

 fourth, the pre-oj)erculum, 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 moditied 

 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 





■/--^ 





''^^^^- 



'^^ 



-SKELETON" 



Pre-niaxillary bone ; ?/, maxillary bone ; f, mandible ; ti, palatine ar^-li ; c, cranium ; ./', inter-oporculnm ; 

 ;;,!/', vertebral column ; ?i, pectoral fin ; i, ventral flu ; fc, ?, dorsal fins ; «i, anal fin; ii, u', caudal flu. 



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

 present, carry aerated Ijlood 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 tlie water from solid 

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

 number of branchi-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 orljit, which is known as a spiracle. These spiracles lead 

 into the pharynx. In Ijampreys there are a numl)er 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. 

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

 the gill-openings. 



