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Important types of circulatory system in herdmania are given below: The circulatory system is well developed and closed type. It includes (1) heart and pericardium (2) blood vessels and (3) Blood (1) Heart and Pericardium: ADVERTISEMENTS: It is a non contractile, elongated and transparent tube about 7.0 cm long and 3.0 mm wide running obliquely below the right gonad. It is closed at both the ends and is filled with a colourless pericardial fluid with corpuscles similar to those of blood. Its wall is thick, made of connective tissue and contains blood sinuses and is internally lined by squamous epithelium. Image Source: s3.amazonaws.com ADVERTISEMENTS: The heart is enclosed within the pericardium and is attached to its wall along its entire length by a thin mesentery like connective tissue flap. It…

The origin and ancestory of modern chordates is not very clear and remains obscure. Because their early ancestors most liably were soft bodied and left no definite fossils. The earliest known fossils of truly vertebrate animals were, freshwater forms abundant during the late Silurian and middle devonian periods. Their fossils are collectively known as ostracoderms. They are placed in the group of jawless animals called Agnatha. Image Source: 3.bp.blogspot.com ADVERTISEMENTS: The oldest fragmentary fossils belonging to ostracoderms occur in the late cambrian and middle ordovician. About the place of origin of chordates an American geologist Chamberlain gave the idea of fresh-water origin of chordates in 1900. This idea was supported by Romer and Homer Smith also. Theories of Invertebrate ancestory of chordates: ADVERTISEMENTS: The invertebrate ancestory of chordates is now…

Important body features of scoliodon are given below: 1. Mouth: It is situated a little behind the apex on the ventral side of the anterior region of the body. Mouth is bounded by upper and lower jaws each bearing 1 or 2 rows of sharply, pointed and backwardly directed teeth. Teeth are adapted for holding and tearing of the prey. Image Source: d.fastcompany.net ADVERTISEMENTS: 2. Nares: Two crescentic apertures, the nares or nostrils are present ventro-laterally and anterior to mouth. They are exclusively olfactory, have no role in respiration, as they are not connected to mouth cavity by internal nostrils. ADVERTISEMENTS: 3. External gill slits: Anterior to each pectoral fin, on either side of the body vertically elongated external gill slits or branchial clefts are present in a series of…