- A large sucker surrounding the mouth, strengthened by an annular cartilage.
- Spine-shaped processes on gill arches
Lampreys are also unique among extant vertebrates in having a median dorsal "nostril", the nasohypophysial opening, but some other fossil vertebrates also display the same structure. It is therefore not diagnostic of lampreys only.
Lampreys are devoid of a mineralized skeleton, although traces of globular calcified cartilage may occur in the endoskeleton.
The head of adult lampreys has relatively large eyes, followed posteriorly by a series of seven, rounded gill openings. Dorsally, there is a translucent pineal spot and, anteriorly to it, a median dorsal "nostril" called the nasohypophysial opening because it is the opening of both the olfactory organ and a blind hypophysial tube including the pituitary gland or hypophysis. This tube is thought to be the remnant of the primitive nasopharyngeal duct (see Hyperotreti). The skin of lampreys is naked and shows large neuromasts of the sensory-line system. The unpaired fins are the dorsal and caudal fins, which are strengthened by numerous, thin cartilaginous radials associated with radial muscles. The tail is slightly hypocercal; that is, the fleshy part containing the notochord is downwardly bent.
The sucker which surrounds the mouth is strengthened by a ring-shaped annular cartilage and bears numerous horny denticles. The depression in the sucker is effected by a complex mechanism which comprises a pumping device, the velum, and a recess of the oral cavity, the hydrosinus. The mouth includes a complex "tongue"-like apparatus which shows some resemblance to that of hagfishes as to its basic mechanism. It bears a series of comb-shaped horny "teeth" which can rotate on the tip of a retractable piston cartilage. The overall resemblance of the "tongue" of lampreys and hagfishes was long regarded as a unique cyclostome character. Considering the current phylogeny, it is now better viewed as independently derived from a basically similar device of the common ancestor to all craniates.
The skull of lampreys is, like that of hagfishes, made up of cartilaginous plates and bars, but it is more complex and includes a true cartilaginous braincase. The gills, although enclosed in muscularized pouches in the adult, are supported by unjointed gill arches (see figure in the Craniata page), which form a "branchial basket". The gill arches lie externally to the gill filaments and associated blood vessels. Lampreys possess, like hagfishes, a very large notochord but, in addition, there are small cartilaginous dorsal arcualia (basidorsals and interdorsals).
The brain has a very poorly developed cerebellum but large optic lobes. The spinal cord is flattened, almost ribon-shaped, yet thicker than that of hagfishes.
The eyes possess a lens, but no intrinsic eye muscles for accomodation. The extrinsic eye muscles are as in extant gnathostomes, except for the superior oblique muscle, which is attached posteriorly in the orbit, instead of anteriorly.
The labyrinth has two vertical semicircular canals, a blind endolymphatic duct, and a number of large ciliated sacs which play a role in equilibrium.
Lampreys undergo a larval development which can last up to seven years. The larval lamprey, or "ammocoetes", has no sucker and poorly developed eyes. Its gills are not enclosed in pouches and it feeds by trapping minute food particles with a strand of mucus produced by the pharynx. Between the mouth and the pharynx, the lamprey larva has a two-valved pumping and anti-reflux device, the velum which, in the adult plays no role in the respiration. The skeleton of the larval head largely consists of a special, elastic tissue, the muco-cartilage, which, during metamorphosis gives rise to a variety of tissues, including true cartilage.
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