Petromyzon

Petromyzon is a jawless, quatic vertebrate (cyclostome). it is commonly known as lamprey and found in North America, Europe, West Africa, Japan, Chile, Australia, New Zealand and Tasmania. The body is elongated and laterally compressed (eel-like). Head, trunk and tail are present. Neck is absent. A buccal funnel or sucker directed downward, is found at the anterior end of the head . it is surrounded by lips supported by cartilage. The lips carry sensory papillae. Mouth lies at the head behind each eye. They are separated by interbranchial septa. They do not open into the pharynx in adult. Anus lies at the base of the tail. Urinogenital opening lies on a papilla behind the anus. Skin is slimy and without scales. Lateral lines are present. Two dorsal fins and an anal fin are present. Pectoral and pelvic fins are absent. Skeleton consists of cartilages, noctochordal and membraneous tissues. Bony skeleton is absent. Red blood corpuscle contains nucleus and haemoglobin. A Sexes are separate. Female lays eggs (oviparous). Fertilisation is external. The development involves a characteristic larval stage called Ammocoete larva . lt is very rnuch like Branchiostma in organisation. The larva has a thin and laterally compressed body. It has an unconstricted notochord, oral hood, endostyle, epipharyngeal groove, peripharyngeal bands and a velum, similar to those in Brcmchiostma. Three species of lamprey found in the northern hemisphere are Petromyzon marinus, P. fluviatilis (river lamprey) and P. planeri (brook- lamprey).