dorsal ftn 



nasohypophyseal opening 



buccal funnel 



caudal fin 



nasohypophyseal 



opening , ' 



tentacles 



branchial opening 

 PETROMYZON OR LAMPETRA 

 eye 



phoryngocutaneous duct {on left side^ 



EPTATRETUS 



opening of slime gland*"^ 



bronchial opening 



y-°-^- 



Figure 2-1. Some external features of the lamprey, top, and hagfish, bottom. 



(Figure 2-1), is at the tip of the snout in the hagfish. There 

 are three pairs of tentacles around the nasohypophyseal 

 opening and the mouth. The mouth is circular, but not 

 distinctly funnel-shaped, and it lacks horny teeth. The body 

 IS smooth and in life is covered with sHme produced in part 

 by the slime glands, whose openings can be seen forming a 

 line along the ventrolateral aspect of the body. 



The eyes are indistinct in the case of Eptatretus, and they 

 cannot be seen in Myxine. There are 10 to 15 pairs of bran- 

 chial openings in Eptatretus, the last on the left side serving 

 also as an opening for a passage from the pharynx known 

 as the pharyngocutaneous duct. Ne.xt to the branchial 

 openings are pores of slime glands. Mvxine has only a single 

 pair of openings since all of the gill pouches (5 to 7 pairs) 

 open together. The left opening also serves the pharyngo- 

 cutaneous duct. The caudal (tail) fin extends anteriorly 

 above and below the caudal lobe of the body. The cloaca 

 is a slit-like opening, splitting the anterior margin of the 

 caudal fin (or anal part of the caudal fin). Anterior to the 

 cloacal aperture, a median fin extends to the posterior 

 branchial openings. 



Chondrichthyes 



The sharks and their relatives are placed in this group of 

 gnathostomes on the basis of their cartilaginous skeletons. 

 In terms of their external anatomy, this group has a skin 

 covered by small, spine-bearing scales (placoid scales); some- 

 times there are larger spine-bearing plates scattered over the 

 body (ray); in some the skin is naked. The holocephalan 

 differs most markedly from the elasmobranchs in having the 

 gill openings covered by a flap or operculum rather than 

 having each pouch opening separately. 



Elasmobranchs ^selachians ond rays) The Dogfish Shark 

 {Squalus) has a long fusiform body with two dorsal fins, and 

 two pairs of lateral fins (Figure 2-2). The tapered posterior 

 end of the body, the caudal peduncle, is turned upward, 

 supporting the upper division of the tail fin. This type of tail 

 is described as heterocercal, or "unequal tail." The dorsal 



fins are preceded by strong spines. The pelvic fins, the pos- 

 terior pair of lateral fins, are partly modified into claspers 

 in the adult male. 



The mouth is a crescent-shaped opening, armed with rows 

 of teeth, and it lies on the underside of the head. In fi-ont of 

 the mouth are the openings of the nasal capsules. These 

 openings are subdivided by a small flap into two parts. The 

 eyes are dorsal to the mouth and large. Behind the eye is a 

 vestigial gill slit, the spiracle, and further back are five pairs 

 of gill openings. 



The anterior part of the body of Raja (Figure 2-2), one of 

 the rays, is flat and triangular in shape; the margins are ex- 

 tended out to form the thick pectoral fins (the anterior pair 

 of lateral fins). Close behind the pectoral fins are the pelvic 

 fins; the anterior part of the basal lobes of these may be thick 

 and fleshy. The tail is long and slim and ends in a very small 

 fin. There are two small dorsal finsjust in front of the caudal. 

 Along the dorsal midline there is a row of large pointed 

 spines. The rest of the body is covered by tooth-like placoid 

 scales, irregular in size but generally larger than those of 

 Squalus. 



The eyes are dorsal and just in front of the spiracles. The 

 snout or rostrum is pointed. The mouth is a transverse ven- 

 tral slit, and the nasal capsules lie in front of it. The open- 

 ing of the nasal capsule is divided by a fold that extends back 

 to the margin of the mouth. Behind the mouth are five pairs 

 of gill openings. 



Holocephalans Chimaera (or Hydrolagus) has a large head; 

 from this the body tapers to the tip of the tail (Figure 2-2). 

 The eye is large and there is a soft bulbous snout. The body 

 is naked and grooved by the lateral-line system. The large 

 openings of the ampullae of Lorenzini, sensory organs, can 

 be observed on the cheeks and snout. The mouth is ven- 

 tral and armed with sharp-edged tooth plates. The gills are 

 covered by a fleshy operculum, which is fused ventrally to 

 the isthmus, the midline bar joining lower jaw and body. 

 The margins of the opercula appear to join across the 

 isthmus. 



A strong spine lies in front of the anterior dorsal fin. Well 



LIVING REPRESENTATIVE VERTEBRATES • 23 



