The basis of classification — body plans

Digestive systems — incomplete (one opening, as in cnidarians and flatworms) or complete (a mouth and an anus).

Circulation — open (blood bathes the organs, as in arthropods and most molluscs) or closed (blood stays in vessels, as in annelids and vertebrates).

The trap: pseudocoelomates — the roundworms — do have a body cavity; it is called 'pseudo' because mesoderm lines only part of it, not because it is absent.

The non-chordate phyla

• ›Octopus, the devil-fish, is a mollusc; the starfish is an echinoderm, not a fish; the silverfish is an insect

The trap: a water vascular system is the mark of echinoderms alone — tube feet driven by water, used for movement and feeding.

Chordates and the vertebrate classes

Three chordate features — a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits, at least at some stage. Subphyla: Urochordata (notochord only in the larval tail — Ascidia), Cephalochordata (notochord for life — Branchiostoma), Vertebrata (the notochord is replaced by a backbone).

• ›the platypus and the spiny anteater are mammals that lay eggs; the bat flies but is a mammal; the whale lives in water but breathes air

The trap: a shark has a cartilaginous skeleton and no air bladder, so it must keep swimming to avoid sinking — the bony fish have the air bladder.

Bihar's animals — the dolphin and the tiger

The Gangetic river dolphin (Platanista gangetica) — a mammal of the Ganga and Brahmaputra river systems, nearly blind, which finds its way and its food by echolocation, and surfaces to breathe every minute or two.

• ›it was declared India's National Aquatic Animal in 2009
• ›the Vikramshila Gangetic Dolphin Sanctuary, on a stretch of the Ganga at Bhagalpur, set up in 1991, is the country's sanctuary for this animal; Bihar's rivers hold a large share of its population
• ›threats: fishing nets, dams and barrages that break the river, pollution, and sand mining

Valmiki Tiger Reserve — Bihar's first tiger reserve, it runs along the Nepal border in West Champaran and joins Nepal's Chitwan National Park across that border.

Other Bihar wildlife to know — the Kanwar lake at Begusarai, a Ramsar wetland for migratory birds; the Bhimbandh sanctuary in Munger.

The trap: the river dolphin is a mammal — it breathes air through a blowhole and suckles its young; it is not a fish.

Points to Remember

• ›levels: cellular, tissue, organ, organ system · symmetry: radial, bilateral · diploblastic (cnidarians) vs triploblastic
• ›acoelomate (flatworms) · pseudocoelomate (roundworms) · coelomate (annelids onward) · open vs closed circulation
• ›Porifera — choanocytes · Cnidaria — cnidoblasts, metagenesis · Ctenophora — comb plates · Platyhelminthes — flame cells · Annelida — metameric, nephridia
• ›Arthropoda — largest phylum, jointed legs, Malpighian tubules · Mollusca — mantle, radula · Echinodermata — water vascular system
• ›chordates: notochord, dorsal hollow nerve cord, gill slits · Urochordata, Cephalochordata, Vertebrata
• ›Chondrichthyes — cartilage, no air bladder · Osteichthyes — bone, air bladder · Aves — pneumatic bones · Mammalia — hair, mammary glands
• ›platypus lays eggs · crocodile — four-chambered heart
• ›Gangetic dolphin — National Aquatic Animal 2009, Vikramshila sanctuary, Bhagalpur 1991 · Valmiki Tiger Reserve, West Champaran · Kanwar lake
• ›Exam tip: for a 'which phylum' question, look for the one structure only that group has — flame cells, cnidoblasts, comb plates, tube feet