Non-chordata and Protochordata — Notes & Type Studies
This section covers the major groups of non-chordates and protochordates included in the B.Sc. 1st Year Zoology curriculum. The notes are organized according to major animal groups and focus on important type studies, structural organization, organ systems and long-answer examination topics.
Special emphasis is given to systematic position, general characters, habit and habitat, morphology, digestive, respiratory, circulatory, excretory, nervous and reproductive systems wherever applicable.
| Organism | Important Feature |
|---|---|
| Vorticella | Bell-shaped, stalked ciliate commonly found in freshwater. |
| Leishmania donovani | Flagellated parasitic protozoan associated with visceral leishmaniasis. |
| Giardia lamblia | Binucleate, flagellated intestinal parasite. |
| Entamoeba histolytica | Causes amoebic dysentery and occurs mainly in the human intestine. |
| Trichomonas vaginalis | Flagellated parasite of the human urogenital tract. |
| Eimeria tenella | Important coccidian parasite causing coccidiosis in poultry. |
| Babesia bigemina | Intraerythrocytic parasite of cattle transmitted by ticks. |
The canal system of Scypha is syconoid. Water passes through the following route:
Obelia is a colonial hydrozoan showing both polypoid and medusoid generations. The colony consists of feeding zooids and reproductive zooids.
| Organism | Importance |
|---|---|
| Fasciola hepatica | Liver fluke of sheep and other mammals. |
| Schistosoma | Blood fluke responsible for schistosomiasis. |
| Taenia | Tapeworm of humans and other vertebrates. |
| Echinococcus | Small tapeworm associated with hydatid disease. |
Hirudinaria, commonly known as the Indian cattle leech, is an ectoparasitic annelid. It possesses anterior and posterior suckers and feeds on the blood of vertebrates.
Palaemon is commonly found in freshwater bodies such as ponds, streams, rivers and lakes. It is an active swimmer and feeds on small aquatic organisms, plant material and organic matter.
The body is divided into a cephalothorax and abdomen. The cephalothorax is covered by a hard carapace, while the abdomen consists of several movable segments ending in the telson.
Palaemon possesses numerous specialized appendages. They are grouped into cephalic, thoracic and abdominal appendages.
There are five pairs of cephalic appendages:
Antennules form the first pair of cephalic appendages. Each antennule consists of a basal protopodite and two flagella. They are mainly sensory and help in touch, chemical perception, balance and orientation.
The antennae form the second pair. Each antenna consists of coxa, basis, a long flagellum and a broad exopod called the scaphocerite. They function in sensation, balance, orientation and detection of water currents.
Mandibles form the third pair of cephalic appendages. They are strong and adapted for biting, cutting and grinding food. Each mandible possesses an incisor process, molar process and mandibular palp.
The first maxillae are small and delicate appendages associated with manipulation and transfer of food toward the mouth.
The second maxillae are broader and possess an exopod. They help in food handling and also assist in creating water currents over the gills.
The thorax bears five pairs of walking legs or pereiopods. The first two pairs are modified for handling food and locomotion, with the first pair generally bearing chelae. The remaining legs mainly assist in walking.
The abdomen bears five pairs of swimmerets or pleopods and the terminal telson is associated with the uropods to form a tail fan. These structures help in swimming, ventilation and reproduction.
The digestive tract is complete and consists of mouth, buccal cavity, oesophagus, stomach, intestine and anus. A well-developed hepatopancreas is associated with digestion and absorption.
The stomach consists of a cardiac region and a pyloric region. The cardiac region receives food while the pyloric region helps in filtration and passage of digested material.
The hepatopancreas is a large digestive gland that secretes digestive enzymes and participates in absorption and storage of nutrients.
Respiration takes place through gills situated in the branchial chambers beneath the carapace. Water is continuously circulated over the gills by movements of the mouthparts and appendages. Oxygen diffuses from water into the blood while carbon dioxide diffuses outward.
The circulatory system is open. A dorsal heart lies within the pericardial sinus. Blood is pumped through arteries into body sinuses and eventually returns to the pericardial region.
Excretion is carried out mainly by paired antennal or green glands located near the base of the antennae. Nitrogenous wastes are removed from the haemolymph and discharged through the excretory pores.
The nervous system consists of a dorsal brain, circum-oesophageal connectives and a ventral nerve cord bearing segmentally arranged ganglia. The brain supplies nerves to the anterior sense organs.
Sexes are separate. The male reproductive system produces spermatozoa, while the female produces eggs. Fertilization may occur externally or in association with egg-bearing structures depending on species and reproductive condition. Developing eggs are commonly carried by the female pleopods.
Well-developed muscles control the movement of the jointed appendages. Walking is mainly performed by the thoracic appendages, whereas swimming is aided by abdominal swimmerets and rapid flexion of the abdomen.
Describe the systematic position, general characters, external morphology, appendages, digestive system, respiratory system, circulatory system, excretory system, nervous system and reproductive system of Palaemon with suitable diagrams.
| Organism | Important Area |
|---|---|
| Periplaneta | General morphology, digestive, respiratory, circulatory, nervous and reproductive systems. |
| Phlebotomus | Vector of leishmaniasis. |
| Culex | Important mosquito vector. |
| Aedes | Important vector of several viral diseases. |
| Sitophilus | Important stored-grain pest. |
Unio is a freshwater bivalve commonly found buried partly in mud or sand at the bottom of ponds, lakes and slow-moving streams.
The shell consists of two lateral valves joined dorsally by an elastic ligament. The outer surface is generally covered by a periostracum, while the inner layers contain calcareous material.
The body consists mainly of a visceral mass, muscular foot and mantle. The mantle forms a mantle cavity that contains the gills and communicates with the external environment through inhalant and exhalant water currents.
The digestive system consists of mouth, oesophagus, stomach, intestine, rectum and anus. Food is mainly obtained by filter feeding. Cilia of the gills generate water currents and help transport food particles toward the mouth.
Respiration occurs through the gills or ctenidia. Water enters the mantle cavity through the inhalant opening, passes over the gills and leaves through the exhalant opening. Gaseous exchange occurs across the thin gill surfaces.
The circulatory system is open. The heart lies in the pericardial cavity and consists of a ventricle and two auricles. Blood is distributed through vessels and tissue spaces before returning to the heart.
Excretion is performed by paired kidneys or organ of Bojanus. They remove nitrogenous wastes from the blood and discharge them into the mantle cavity.
The nervous system consists of paired cerebral, pedal and visceral ganglia connected by nerve cords. The cerebral ganglia are associated with the anterior region, pedal ganglia with the foot and visceral ganglia with the posterior visceral region.
Sense organs are comparatively simple. Sensory cells occur in the mantle, siphonal region and foot. Statocysts associated with the pedal ganglia assist in maintaining equilibrium.
The foot is a large muscular structure that projects between the shell valves. It is used mainly for burrowing into mud or sand. Contraction and extension of foot muscles allow slow movement.
Sexes are generally separate. The gonads are situated in the visceral mass. Fertilization is generally internal or occurs within the mantle cavity, and development may involve a larval stage such as glochidium in unionid mussels.
Pearl formation is a defensive response of the mollusc to an irritating foreign particle present between the shell and mantle.
Pearls are valuable ornamental materials and have considerable economic importance in jewellery and decorative products.
Achatina is a large terrestrial gastropod commonly known as the African giant land snail. It is adapted to terrestrial life and possesses a well-developed shell, muscular foot and pulmonary respiratory organ.
It is mainly terrestrial and occurs in moist environments, gardens, agricultural areas and regions containing abundant vegetation.
The body is divided into head, foot and visceral mass. The head bears tentacles and the mouth. The muscular foot helps in creeping. The visceral mass is enclosed by a spiral shell.
The digestive system consists of mouth, buccal cavity, oesophagus, stomach, intestine and anus. The radula is used for scraping and cutting food.
The mantle cavity is modified into a pulmonary chamber. Air enters through the respiratory opening and gaseous exchange occurs across the vascularized wall.
Achatina is hermaphroditic, meaning both male and female reproductive organs occur in the same individual. Cross- fertilization commonly occurs between individuals.
Achatina is a terrestrial gastropod characterized by a spiral shell, head with tentacles, muscular foot and pulmonary respiration. It is adapted to terrestrial life through shell protection, mucus secretion and a pulmonary chamber. It is hermaphroditic and generally reproduces by cross-fertilization.
Asterias, commonly called a starfish, possesses a central disc and radiating arms. The oral surface contains the mouth, while the aboral surface contains the anus and madreporite.
Balanoglossus is important in zoology because it shows several structural features associated with the transition between invertebrate and chordate organization.
Herdmania is a marine urochordate commonly known as a sea squirt. The adult is sessile and filter-feeding, whereas the larva is free-swimming and possesses several important chordate characteristics.
The affinities of Herdmania are important because the adult has a highly modified sessile body, whereas the larva clearly expresses several characteristic chordate features.
The larval tail contains a notochord. It is mainly restricted to the tail region and disappears during metamorphosis.
The larval nervous system includes a dorsal tubular nerve cord, which is a characteristic chordate feature.
The pharynx contains numerous gill slits. These are associated with filter feeding and respiration.
An endostyle is present in the pharyngeal region. It secretes mucus that helps trap food particles from the water current. The endostyle is considered homologous with the thyroid gland of vertebrates.
The larva possesses a post-anal tail containing the notochord and nerve cord.
The larval chordate organization represents primitive chordate features, while the highly specialized adult pharynx and sessile filter-feeding habit represent adaptations associated with the adult mode of life.
The most important evidence for the chordate affinity of Herdmania is found in the larva, particularly the notochord, dorsal hollow nerve cord, pharyngeal gill slits, endostyle and post-anal tail.
Herdmania occupies an important position in chordate studies. Its adult body is highly specialized for a sessile filter-feeding mode of life, while its free-swimming larva retains major chordate characteristics. Therefore, the larval features are especially important for understanding its relationship with other chordates.
Discuss the affinities of Herdmania with suitable reference to its chordate characters, urochordate features, larval organization and specialized adult condition.
Branchiostoma retains the fundamental chordate features throughout life and is therefore useful for understanding the basic organization of chordates.
| Feature | Herdmania | Branchiostoma |
|---|---|---|
| Subphylum | Urochordata | Cephalochordata |
| Adult habit | Generally sessile | Free-living and active |
| Notochord | Mainly present in larval tail | Extends throughout body and persists in adult |
| Body covering | Tunic or test | Thin body wall |
| Pharynx | Highly developed | Highly developed |
| Post-anal tail | Present in larva | Present throughout life |
For long-answer questions, prepare each major type study using a consistent sequence. This makes the answer organized and helps ensure that important points are not missed.
For long questions, always practice at least one neat, well-labelled diagram for the major type study. The diagram should have a clear title and important structures should be labelled without overcrowding.