Two living groups of jawless fishes, the lampreys (Petromyzontida) and the hagfishes (Myxinoidea), lie close to the base of the Vertebrata, and they may share more primitive features with their Cambrian forebears than the other Palaeozoic fishes. Lampreys and hag-fishes are very different from many of the extinct jawless fishes, but they are unique in perhaps showing us something of the Early Palaeozoic world, before jaws existed. Both groups have elongated bodies, no bony armour, no jaws and no paired fins.
The 30 or so species of lampreys all spend some of their life in freshwaters where they breed. Most are parasitic, and they feed by attaching themselves to other fishes with their sucker-like mouths (Figure 3.4(a)), and rasping at the flesh. The mouth and oesophagus are within a deep funnel, which is lined with small pointed teeth that permit firm attachment to the prey. There is a fleshy protusible 'tongue', which also bears teeth and which is used in rasping at the flesh. Lampreys (Figure 3.4(b)) have a single nasal opening on top of the head that runs into a pouch beneath the brain, large eyes and two vertical semicircular canals in the internal ears on each side. There is an internal skeleton consisting of a notochord, vertebra-like structures, an attached cartilaginous skull and gill arches, and fin rays.
The marine hagfishes (Figure 3.4(c)) look superficially like lampreys, but they live in burrows in soft sediments, feeding on invertebrates and decaying carcasses on the sea-bed. Hagfishes have a single nasal opening at the very front that connects directly to the pharynx (Figure 3.4(d)), quite unlike the lamprey nostril. The
cloaca openings dorsal fins cloaca openings dorsal fins notochord spinal cord oesophagus / brairi mouth heart spinal cord oesophagus / brairi mouth notochord heart
eye spot eye spot cloaca openings of gill exits slime glands cloaca openings of gill exits slime glands
notochord nasopharyngeal duct gill pouches 10StrM
horny teeth hypophysial pouch notochord nasopharyngeal duct
mouth pharynx teeth on tongue id)
mouth pharynx teeth on tongue
Fig. 3.4 Living jawless fishes: (a) lamprey, feeding by attachment to a bony fish, and (b) longitudinal section of anterior end of body; (c) Pacific hagfish, external lateral view ofbody, and (d) longitudinal section of anterior end of body. (Modified from Young, 1981, and other eyes are reduced and often covered with thick skin and muscle, and there is only one semicircular canal on each side. The mouth is ringed with six strong tentacles, and inside it are two pairs of horny plates bearing numerous small keratin toothlets that can be protruded with the mouth lining. This apparatus can be turned in and out, producing a pinching action with which the hag-fishes can grasp the flesh of a dead or dying animal. They remove a large lump of flesh by holding it in a firm grasp, and tying a knot in the tail, passing it forwards towards the head, and bracing against the side of their prey.
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