Introduction
As noted in Chapter 1, the earliest craniates lacked jaws, and so are termed “agnathans.” Numerous extinct “agnathans” are known from the fossil record and most possessed a covering armor of dermal bone, from which is derived the term “ostracoderms.” Living craniates retaining the absence of jaws are the hagfishes (Myxi- noidea) and lampreys (Petromyzontoidea).
A characteristic feature of these craniates is an anterior, rounded, sucker-like structure used to attach themselves to the body of their prey, from which the term “cyclostome” is derived. The relationship between these “agnathans” is not settled. They were for many years considered to be each other’s closest relatives and classified as “Cyclostomata.” About two decades ago, morphologists began to note that some characteristics of lampreys indicated they were more closely related to the jawed vertebrates and classified with them in the Vertebrata. Most recently, molecular evidence is again suggesting a close relationship between “cyclostomes.”Of the approximately 50 lamprey species, the marine lamprey, Petromyzon marinus, of the Atlantic Ocean and North American Great Lakes, is the most commonly studied “cyclostome.” Hagfishes are about as diverse, with 60 species generally recognized, and are exclusively marine—indeed, they are the only vertebrates having their body fluids isosmotic with seawater. While lampreys and hagfishes share several similarities, such as an eel-like body shape and a sucker-like mouth, there are important differences between them. Lampreys are generally parasitic, attaching themselves to their prey and relying mainly on a liquid diet, while hagfishes are scavengers and tear off pieces of dead or dying prey. These differences are reflected in several innovative modifications. The lamprey, for example, has a subdivided pharynx, with the ventral part forming a respiratory tube that can be isolated from the mouth by a valve termed the velum. This ensures that its liquid diet neither escapes from the pharyngeal slits nor interferes with gas exchange through the gills. While the tube is isolated, the lamprey continues to ventilate its gills by pumping water in and out of the pharyngeal slits. The hagfish, on the other hand, has a more substantial diet and does not require a respiratory tube that can be isolated. It ventilates its gills by having a nasal opening that continues past the nasal sac to communicate with the pharynx. A velum is also present in the hagfish. With the velum closed, muscular action compresses the pharynx and water moves over the gills.