GENERAL STUDY OF TYPICAL CHORD ATES 21 



intestine and urogenital systems have separate openings; this is one of the marked 

 differences between elasmobranch and teleost fishes. 



H. SOME GANOID FISHES 



As certain ganoid fishes are frequently referred to in the study of the skeleton, 

 it is advisable that the student become familiar with their appearance. The 

 ganoid fishes belong to the lower and more primitive orders of the Teleostomi 

 and hence are placed in the scheme of classification below the teleosts, but the 

 latter have been considered first to introduce the terminology. We shall examine 

 briefly four common ganoids. 



1. The gar pike (Lepidosteus) . — The body is clothed in a complete armor 

 of rhomboid scales set in diagonal rows. Such scales are called ganoid scales 

 and consist of bone with an outer coat of a shiny substance named ganoin. 

 These scales pass onto the head as enlarged plates, which constitute the bones of 

 the roof of the skull. The jaws are very elongated, producing a snout on the 

 tip of which are the small nostrils. The jaws bear numerous sharp teeth. There 

 is an operculum covering the gills. A spiracle is absent. Note median and 

 paired fins, with their stout, jointed fin rays. The tail is heterocercal but 

 approaches the homocercal type. 



2. The sturgeon (Acipenser). — There is a covering of ganoid scales arranged 

 in five rows on the trunk with areas of apparently naked skin (which really bear 

 smaU scales) between the rows. The scales lack the outer coat of ganoin. Most 

 of them bear sharp posteriorly directed spines. They pass onto the head, 

 transforming into skull bones. The head is broad and terminates in a large 

 rostrum, bearing many small scales on its dorsal surface. The nostril is divided 

 by a partition into two openings. The operculum contains a single bone. There 

 is a slitlike spiracle above each eye. On the ventral side of the rostrum are four 

 branched processes, the barbels, used as sense organs for the detection of food. 

 The mouth is curious in form, distensible, and with sensory papillae on its 

 borders. The jaws are degenerate, and there are no teeth. Note paired and 

 median fins; the tail is heterocercal. 



3. The spoonbill (Polyodon). — In this curious fish the rostrum is expanded 

 into a broad, thin, spatulate structure, provided with sense organs for the detec- 

 tion of food. At the base of the rostrum just in front of the eyes are the nostrils, 

 each with two openings. Behind the eyes is the small spiracle. The operculum 

 contains no opercular bones and is extended into a pointed process; the branchio- 

 stegal membrane, continuous with the opercula, has no branchiostegal rays. 

 Lift up the operculum and note the numerous long and fine gill rakers on the gill 

 arches, used by the fish in separating food particles from mud. The jaws open 

 widely and are provided with minute teeth. The body is naked. Note the fins. 

 The tail is typically heterocercal. The fish is singularly lacking in external 

 hard parts. 



