Zoology Cheatsheet

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Basic Characteristics of Chordates Notochord: A flexible, rod-like structure providing skeletal support, present at some stage of life. In most vertebrates, it is replaced by the vertebral column. Dorsal, Hollow Nerve Cord: Located dorsally to the notochord, it develops into the brain and spinal cord in vertebrates. Pharyngeal Slits: Openings in the pharynx that may develop into gills or other structures involved in feeding or respiration. Post-Anal Tail: An extension of the body posterior to the anus, which contains skeletal elements and muscles, used for locomotion in many species. Endostyle/Thyroid Gland: A ciliated groove in the pharynx floor, involved in filter feeding, which is homologous to the thyroid gland in vertebrates. Bilateral Symmetry: Body can be divided into two mirror-image halves. Triploblastic: Possess three germ layers (ectoderm, mesoderm, endoderm). Coelomate: Possess a true coelom (body cavity). General Features of Protochordates (Acrania) Protochordates are an informal group of invertebrates that are closely related to vertebrates. They share the basic chordate characteristics but lack a true head and vertebral column. Marine Habitat: Exclusively found in marine environments. Lack Vertebral Column: Notochord persists throughout life and is not replaced by vertebrae. No Paired Appendages: Generally lack fins or limbs. Simple Nervous System: Dorsal nerve cord is present but lacks a complex brain. Filter Feeders: Most are sessile or slow-moving filter feeders. Absence of Jaws: Mouth is typically simple and jawless. Examples: Urochordata (Tunicates): Ascidians (sea squirts), Salps. Larval stage exhibits all chordate features; adults are often sessile with a degenerate notochord. Cephalochordata (Lancelets): Amphioxus (e.g., Branchiostoma ). Possess all chordate features in a more prominent and persistent form throughout their life cycle. Morphology of Balanoglossus (Acorn Worm) Balanoglossus is a genus of acorn worms (Hemichordata), which are marine invertebrates often considered a link between invertebrates and chordates due to their possession of pharyngeal gill slits and a rudimentary notochord-like structure (stomochord). Body Divisions: The body is distinctly divided into three regions: Proboscis (Protostome): Anterior-most, muscular, conical or acorn-shaped structure used for burrowing and food collection. Contains the buccal diverticulum (stomochord). Collar (Mesostome): A short, cylindrical region behind the proboscis. It bears the mouth on its ventral side and includes the coelomic cavity. Trunk (Metastome): The longest part of the body, cylindrical and somewhat flattened. It is further divided into: Branchiogenital Region: Anterior part of the trunk, characterized by numerous paired gill slits (pharyngeal slits) for respiration, and genital wings containing gonads. Hepatic Region: Middle part of the trunk with a series of dorsal sacculations (hepatic caeca) that contain digestive glands. Post-hepatic/Caudal Region: Posterior part, which is plain and tapering, ending in the anus. Skin: Covered by an epidermis with mucous glands, giving it a slimy appearance. Coelom: Divided into three distinct parts corresponding to the body regions (proboscis coelom, collar coelom, trunk coelom). Mouth and Anus: Mouth is ventral to the collar, anus is terminal at the posterior end of the trunk. Coloration: Varies by species, often yellowish, brownish, or reddish. Size: Can range from a few centimeters to over two meters in length. Characters of Cyclostomes with Examples Cyclostomes are a group of jawless fish (Agnatha) that represent the most primitive living vertebrates. They include lampreys and hagfishes. Jawless (Agnatha): Lack true jaws; mouth is round and suctorial, adapted for grasping prey or attaching to hosts. Eel-like Body: Elongated, cylindrical, and scaleless body. Cartilaginous Skeleton: Endoskeleton is entirely cartilaginous; notochord persists throughout life. Vertebrae are rudimentary or absent. Unpaired Fins: Possess median fins (dorsal and caudal) but lack paired fins (pectoral and pelvic). Gills: Have 5-16 pairs of gill slits for respiration, located in separate pouches. Single Nostril: Possess a single median nostril. Two-Chambered Heart: Simple circulatory system with a heart composed of an atrium and a ventricle. Cold-Blooded (Poikilothermic): Body temperature varies with the environment. External Fertilization: Reproduction typically involves external fertilization. Examples: Lampreys (e.g., Petromyzon , Lampetra ): Parasitic or non-parasitic. Mouth is a suctorial funnel with horny teeth. Possess seven pairs of gill slits. Many species undergo metamorphosis from an ammocoete larva. Hagfishes (e.g., Myxine , Eptatretus ): Scavengers, primarily feeding on dead or dying fish. Mouth is surrounded by barbels (sensory tentacles). Produce copious amounts of slime as a defense mechanism. Possess 5-16 pairs of gill pouches.