branchial openings 



spine 



SQUALUS ACANTHIAS 



clospers 



operculum . ' . r i l 



opening of closper pouch 



HYDROLAGUS 

 Figure 2-2. External features of the chondrichthians: shark, ray, and chimaerid. 



behind this are two more low dorsals and a caudal fin that 

 tapers to a point. It has equally developed dorsal and ven- 

 tral fin lobes and is thus of a type called isocercal. The 

 pectoral fins are large and wing-like; the pelvics lie antero- 

 lateral to the anal opening. The pectoral and pelvic fins 

 both have distinct fleshy basal lobes. In the adult male the 

 pelvics have anterior and posterior claspers. The anterior 

 clasper is sheathed in a pouch below the anterior margin of 

 the base of the fin; the posterior one extends straight back 

 from the inner margin of the fin. These claspers and that on 

 the head are armed with sharp, hooked denticles. 



Actinopterygians 



The chondrosteans, holosteans, and teleosts lack a distinct 

 fleshy lobe at the base of the fins. The fin is membranous 

 with supporting, jointed rays radiating from the base. The 

 rays branch as they e.xtend outward and the tips taper at 

 the fin margin. From this kind of fin, this association of 

 fishes gets its name, the ray-finned fishes. 



Chondrosteans Aapenser, the sturgeon, Scaphirhynchus, the 

 Shovel-Nosed Sturgeon, and Polyodon, the Paddlefish, are 

 all representatives of this group. (Figure 2-3). These fishes 

 resemble the sharks in the development of a rostrum. The 

 top of the head is covered by bony plates, and a large plate 

 lies in the operculum of either side. The eye is dorsolateral, 

 and in front of it are the two openings of the nasal capsule— 

 a small dorsal opening and a larger posterior ventral one. 



Behind the eye but in front of the opening of the operculum 

 is a small spiracle (lacking in Scaphirhynchus). 



The mouth is far back on the underside of the rostrum. 

 In the sturgeons there are four sensory feelers (barbels) in 

 front of it, two in Polyodon. The lips of the sturgeons also 

 have clumps of small sensory papillae on them. The bran- 

 chial (gill) chambers open by a slit behind the operculum. 

 The body of the sturgeons has dorsal, lateral, and ventro- 

 lateral rows of large, spine-bearing plates (scutes). Between 

 these are small, bony rhomboid scales, each of which has 

 a spine. These scales are named for their shape. There is a 

 band of articulated rhomboid scales on the fleshy lobe of 

 the caudal fin. The caudal peduncle of Scaphirhynchus is 

 armored with scutes. 



There is a single dorsal and anal fin, and there are two 

 pairs of lateral fins with fleshy basal lobes. The tail is heter- 

 ocercal; its front margin is formed of spines called fulcra. 

 The anterior margin of the dorsal fin may also be formed 

 partly of fulcra. 



Polyodon differs from the sturgeons in that it lacks the 

 rows of scutes on its body; the caudal fin has fulcra and there 

 are rhomboid scales on the caudal lobe. The body is naked 

 except for minute elements enclosing the lateral-line canal. 

 This canal has short canals leading dorsally and ventrally 

 to pores. The spatulate rostrum is the most noteworthy 

 feature. The eye is at the base of this bill, the mouth is large 

 and toothless, and there is a spiracle midway between the 

 eye and the dorsal margin of the opercular fold. The oper- 

 culum extends far back on the body as a pointed flap. 



24 • THE VERTEBRATES AND THEIR EXTERNAL ANATOMY 



