— Lancelets (Amphioxus / Branchiostoma)
| General Features | Exclusively marine animals, superficially like fish. Capable of swimming, but are usually buried in sand with only their anterior end exposed. Body small and fish like, metameric and transparent. Tail is present throughout life. All chordate characteristics are present throughout their life history (notochord, dorsal tubular nerve cord, pharyngeal gill slits). Notochord extends forward inside the rostrum, even beyond the brain region. Both larvae and adults are motile. Direct development in which a free swimming larval stage gradually changes into an adult. Body cavity is a true coelom, lined with mesoderm and filled with lymph like coelomic fluid. |
| Body Structure | Lacks bones or brain. True head is absent. Two divisions of the body: larger anterior Trunk, and the post anal tail. Anterior of the trunk projects as a pointed rostrum. Trunk has 3 openings: Mouth, Atriopore, Anus. Oral hood is a denticulated structure below the rostrum. Atrial cavity surrounding the Pharynx opens to the outside through the Atriopore. Anus is at the base of the Caudal Fin. 3 longitudinal median, unpaired fins. Dorsal and Ventral fins are internally supported by one and two rows of small rectangular fin-ray boxes respectively. Fin rays are lacking in caudal fin. Structure of fins and fin-ray boxes is different from that in fishes. Lateral section has V shaped Myotome and Gonads. |
| Body Covering | Outer epidermis is ciliated in young individuals, but in mature individuals, the cilia is lost and cuticle is secreted externally. Beneath the epidermis is a strong, thin and compact fibrous tissue layer called Cutis. Epidermis is simple and similar to that of invertebrates (stratified in all higher chordates) |
| Muscles | Distinct metameric segmentation. Arranged along the length of the body in a series of V shaped muscle blocks called Myotomes. Separated from adjacent myotomes by myosepta. Myotomes help in lateral undulating movements of the body as they can be twisted sidewise rapidly. |
| Skeleton | No exoskeleton. Endoskeleton structures are present but they are neither bony nor cartilaginous. Notochord is the main endoskeleton structure. Fin rays are formed from modified connective tissue containing gelatinous substance. |
| Digestive System | Alimentary canal is a straight and complete tube from mouth to anus. Lined throughout by ciliated epithelium. Pharynx forms the largest part of the alimentary canal. Pharyngeal slits are numerous and better developed. Midgut diverticulum is the main digestive gland. Analogous to the Liver, it is the main digestive gland. Epithelial lining of intestine also contains gland cells. Feeding Filter feeders Inside of the oral hood is lined with cilia — wheel organ. Cilia in the mouth in the wheel organ and in the pharynx help generate a water current. Water and suspended food particles pass through the oral hood, equipped with projections called cirri that strain larger particles, and pass out of the body through the Atriopore. In the enterostome, water current is further filtered by the velar tentacles, so that only very fine food particles enter the pharynx. Food particles that escape the main current are caught up and concentrated by the mucus secreted by Hatschek’s groom and pit in the oral hood. They are pushed through enterostome into the pharynx by whirling action of wheel organ, and join the main current. Ingress of water into the pharynx is controlled by the velum. Rejection current: Atriopore closes and transverse muscles of the atrial floor undergo sudden contraction. Sets up a reversal of current ejecting out forcefully through the enterostome and mouth. Allows velar tentacles and buccal cirri to shed off larger food or sand particles. Pharynx plays the most important role. Outward beating of lateral cilia on gill bars and atrial cilia on outer surface drive water current through gill slits into the atrium, and then out through atriopore. Inside, food particles are trapped by mucus secreted by glands. Food laden with mucus is moved into the oesophagus through ciliary movements, in form of a narrow food cord. Digestive enzymes are secreted from midgut diverticulum and midgut epithelium. Food cord from pharynx passes through oesophagus into midgut. Lateral tract of cilia directs it into midgut diverticulum and again drives it to the midgut. Digestion starts in the midgut and continues in the intestine. Digested matter is absorbed partly in the midgut but mostly in hind gut. Undigested material is removed through anus. |
| Respiratory System | No special respiratory organs. Pharyngeal wall is richly vascular, and the water in the pharyngeal current brings in oxygen. Exchange of Oxygen and Carbon Dioxide occurs across this surface. |
| Circulatory System | Closed type, well developed. Blood is colourless — no pigment, no corpuscles. No heart. All vessels are muscular and contractile. |
| Nervous System | Hollow dorsal nerve cord, just above the notochord. No ganglia. Paired nerves arise from the cord. Central canal called Neurocoel runs throughout the length and is filled with a cerebrospinal spinal fluid. Slight enlargement near the rostrum — so called brain. |
| Excretory System | Nephridia develop from the ectodermal cells, compared to kidney of vertebrates which are mesodermal in origin. Proto-Nephridia are simple, closed, ciliated, sac-like and thin-walled ectodermal tubules. |
| Reproduction | Sexes are separate but alike externally. Fertilisation is external. Gonads are mesodermal in origin and bulge into the atrial cavity. Gametes are ejected through the atriopore. |
| Primitive Characters | Specialised Characters |
|---|---|
| Asymmetrical body like echinoderms. No specialisation of head or cephalisation. No paired limbs or fins. Epidermis one cell thick. Dermis absent. Metamerically arranged muscles (myotomes). Notochord persistent throughout life. Vertebral column or any other endoskeleton is not developed. Jaws absent. Alimentary canal straight. Pharynx specialised for ciliary mode of feeding. Liver represented by the midgut diverticulum. Blood vascular system is simple. No heart. Hepatic portal system is primitive. No special respiratory organs or pigments. Excretion by segmentally arranged proto-nephridia. Neural tube hollow, no specialised brain. Dorsal roots without ganglia.. Eggs are small, almost yolk-less. | Asymmetry of adult and early stages of development. Anterior projection of notochord into the rostrum, making it stronger for burrowing. Overdevelopment of notochord may be responsible for lack of brain. Mouth surrounded by oral hood, with sensory oral cirri, for filtering and concentrating food particles from water. Elaborate velum with sensory tentacles to permit only small food particles to enter the pharynx. Degenerate Characters (degeneration through evolution from a more specialised organism). Lack of gonoducts. Lack of cartilaginous or bony endoskeleton. Poorly developed brain and simple sensory organs. Ciliated gill clefts which allow sufficient food collection. Wheel organ and Hatschek’s groove developed to help in ciliary feeding. Protective atrial cavity to outside through atriopore. |
Affinities
Affinities to non chordates
Annelida: Bilaterally symmetrical, metamerically segmented body; well developed coelom; closed blood vascular system; filter feeding.
Mollusca: Filter feeding.
Echinodermata: Asymmetrical body; perforations in calyx of fossil echinoderms resemble gill slits.
Affinities to Chordates
-Hemichordata
Similarities: Pharyngeal apparatus with numerous gill slits, filter feeding mechanism, respiratory mechanism, numerous gonads without gonoducts.
Differences: In Hemichordata, muscles are unsegmented, nervous system of non-chordate type, post-anal tail is lacking, notochord is not a true notochord.
-Urochordata
Similarities: Primitive ciliary feeding and respiratory mechanisms, large pharynx with numerous lateral gill slits, ectoderm lined atrial cavity opening externally through an atrial pore, identical early stages of development, continuous notochord, post anal tail.
Differences: Body segmented, covering test, well developed muscular heart, presence of liver, no nephridia, hermaphrodite glands and united sexes, retrogressive metamorphosis.
-Cyclostomata
Elongated, slender, fish like body; continuous dorsal median fin, mouth surrounded by an oral hood, guarded by a velum, pharynx with endostyle and gill slits.
-Other Vertebrates
Similarities: Metamerically arranged myotomes, true coelom lined by mesodermal epithelium, post-anal tail, midgut diverticulum comparable to liver, well formed hepatic portal system.
Differences: Lack of head, paired limbs, vertebral column, muscular heart, specialised sense organs.
Presence of nephridia, atrium, numerous gonads.