Class Chondrichthyans - NEET - Biology

Class Chondrichthyans

Fig: Rhinocodon

• Chondrichthyans = cartilage fish.

• They have a skeleton that is composed predominantly of cartilage, often impregnated with calcium.

Important characteristics:

• Marine animals having cartilaginous endoskeleton.

• The notochord is persistent throughout life.

• Gill slits are separate & operculum (gill cover) is not present.

• The skin is tough, containing minute placoid scales.

• The mouth is located ventrally. Jaws are present. Teeth are modified placoid scales. 

• The stomach is J-shaped.

• The swim bladder & lungs are absent and the liver is filled with oil to provide buoyancy to the body while swimming.

• Due to the absence of an air bladder (fish maw), they have to swim constantly to avoid sinking. If they stop swimming, they will sink (fall) like a stone.

• The heart is two-chambered (one auricle & one ventricle).

• Kidneys opisthonephric. Excretion ureotelic. Cloaca present.

• They are cold-blooded animals, i.e. they are incapable to regulate their body temperature. They do not maintain constant body temperature. They get their heat from the outer environment.

• Fertilization is external. Sexes are separate.

• The pectoral fins of the shark are called claspers & used for copulation.

There are three ways in which sharks reproduce;

• Oviparous= in which the female lays eggs which takes a few months to develop.

• Ovoviviparous= where the eggs are hatched in the oviduct & the embryo develops in the uterus.

• Viviparous= in which the gestation period of the embryo is about one year.

Examples: Rhinocodon (whale shark), Carcharodon (great white shark), Trygon (stingrays), Torpedo (electric rays), Scoliodon (dogfish), Pristis (sawfish).

Fig: Stingrays

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Class Reptilia- NEET-Biology

 Class Reptilia

Fig: Sphenodon (Tuatara)

• Reptilia= Creeping or crawling mode of locomotion.

• It is considered the first animal on the land with the ability to live & multiply on land, with the help of their amniotic eggs.

• Most of them are tetrapods, with four-legs or like-like appendages.

Important characteristics:

• They are mostly terrestrial animals & their body is covered by dry & cornified skin.

• The skin is covered with scutes or scales & it has a high level of keratin, which prevents water loss through the skin.

• Glands are usually absent.

• Snakes & lizards shed their scales as skin cast (sloughing) routinely.

• They are considered tetrapods with sets of paired limbs. In some reptiles, like snakes, worm lizards, the legs are absent, but it is believed that these animals evolve from some tetrapod ancestor.

• Unlike amphibians, reptiles do not pass through an embryonic stage with gills. These animals breathe with well-developed lungs, right from birth. Most of them have two lungs, except for some snakes, which posses only a single lung.

• All reptiles have three-chambered hearts, except crocodiles, which have four-chambered (2 atria, 2 ventricles), like mammals & birds. The three chambers in reptiles consist of two atria to receive blood & one partially divided ventricle for pumping blood.

• Reptiles do not have external ear openings. The tympanum represents the ear.

• Reptiles are the first animals with amniotic eggs, that can be laid on land & not in water. These amniotic eggs can survive without water, as they have protected shells & membranes, that allow oxygen & other gases to enter.

• Most of the reptiles lay eggs, but some of them give birth to young ones, by hatching the eggs inside the body of the mother.

• Their characteristics also include internal fertilization, in this process sperm gets deposited into the reproductive tract of the female directly.

• Being cold-blooded, the body temperature of the reptiles vary with the surrounding atmosphere.

• Sexes are separate. Fertilization is internal.

• They are oviparous & development is direct.

• Example: Sphenodon (Tuatara), Varanus (Komodo dragon), Draco (Flying lizard), Ophiophagus (King cobra), Hydrophis (Sea snake), Crocodylus (Indian freshwater crocodile).

Fig: Ophiophagus (King cobra)

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Phylum - Porifera - NEET-Biology

 Phylum Porifera

Fig: Phylum Porifera = Pore Bearing

Important Characteristics:

• Phylum Porifera = Pore-bearing animals.

• Sponges are primitive multicellular animals with a cellular grade of organization. 

• They have no fixed body shape & no plane of symmetry. 

• Whole sponges can be regenerated from a few separated cells.

• Sponges can almost be regarded not as individuals but as colonies of separate cells.

• Sponges are free-living aquatic (mostly marine) & having neither nerves nor muscles.

• Body wall with two layers of loosely arranged cells and mesenchyme in between (diploblastic).

• Reproduction occurs by asexual (external & internal buds) or sexual methods.

• The body-wall encloses a large cavity, the spongocoel & in most cases also contain numerous small canals. 

• Either the spongocoel or canals are lined by special collar cells or choanocytes with flagella.

• The ceaseless beating of flagella maintains s steady current of water through the canals to bring in food & oxygen & remove waters.

• Almost all sponges possess an internal skeleton. It may consist of tiny siliceous spicules or of fine spongin fibers or of both.

• Example: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge)

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Phylum Platyhelminthes (Flatworms) - NEET-Biology

 Phylum Platyhelminthes (Flatworms)

Important Characteristics:

• Acoelomate, triploblastic, bilaterally symmetrical organisms.

• Possess a mouth but no anus- Blind sac body plan.

• Digestive, skeletal, circulatory & respiratory systems are absent, parenchymal gland serves as hydroskeleton.

• The body is soft & dorso-ventrally flattened. It may be leaf-like ribbon-like. It is without segmentation.

• Possess the organ-system level of organization.

• The organ-system level of organization.

• The organ-system is packed in a characterize connective tissue, called parenchyma.

• The nervous system is ladder-like. It comprises the brain & two main longitudinal nerve-chords connected at intervals by transverse commissures.

• The excretory system includes characteristics of flame-cells leading into tubules that open out by one or more excretory pores.

•  Hermaphroditic, often with elaborate precautions for minimizing self-fertilization.

• Fertilization is internal. Life history often includes larval stages.

• Asexual reproduction by transverse fission occurs in some forms.

• Example- Taenia (Tapeworm), Fasciola (Liver fluke)

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Phylum Annelida - NEET-Biology

 Phylum Annelida

Fig: Leech (Hirudinaria)

Characteristics:

• Triploblastic, coelomate, bilaterally symmetrical & metamorphically segmented animals.

• In earthworm, each segment has a separate coelom which acts as a hydraulic skeleton.

• During muscular contraction, the body wall pushes against each compartment wall. This allows separate regions to contract independently & elongate during locomotion.

• The body is elongated, cylindrical, or flattened.

• It is divided into segments by ring-like grooves, the annuli. This character gives the phylum its name.

• The segmented worms are triploblastic, i.e. they develop from the three germ layers.

• The body-cavity is a true coelom, as it is lined by a mesodermal epithelium.

• It is divided by vertical septa into compartments.

• A closed circulatory system present.

• Segmented nephridia for excretion & osmoregulation.

• Typically, there is a trochophore larva during development.

• Example: Nereis, Pheretima (Earthworm), & Hirudinaria (Bloodsucking leech).

Fig: Earthworm (Pheretima)

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Phylum Arthropoda -Biology

Phylum Arthropoda

Fig: Apis (Honey Bee)

Important Characteristics:

• It is the largest phylum of Animalia.

• Exhibit organ-system level of organization.

• Bilaterally symmetrical, triploblastic, segmented & coelomate animals.

• Metamorphic segmentation as in annelids but segmented not separated from each other by septa.

• All arthropods have an exoskeleton of a chitinous cuticle. 

• Cuticle shed periodically to allow for growth.

• The process of casting off of skin or integument is known as ecdysis or molting.

• The body is divisible into the head, thorax & abdomen or divisible into cephalothorax and abdomen.

• They have jointed appendages (arthos-joint, poda- appendages).

• Arthropods have an open circulatory system. The body cavity is known as hemocoel (hemocoel), is filled with fluid hemolymph.

• Respiration takes place through the general body surface & gills in crustacean; through trachea in insects, Diplopoda, Chilopoda; through book lungs in Arachnida.

• Excretion is brought about by green glands in aquatic forms & Malpighian tubules in terrestrial forms.

• Sexes usually separate; Fertilization internal; oviparous or ovoviviparous; development direct or indirect.

• Example: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect).

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Phylum Mollusca - NEET-Biology

 Phylum Mollusca

Fig: Apple snail (Pila)

Characteristics:

• Mollusca is the second-largest animal phylum & includes snails, slugs, oysters, cuttlefish, octopuses, and many other familiar animals.

• Exhibit organ-system level of body organization.

• Bilaterally symmetrical, triploblastic & coelomate animals.

• The body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot & visceral hump.

• Most mollusks have a rasping tongue-like radula for feeding.

• Except for cephalopods, all mollusks have such as open circulatory system.

• Nitrogenous wastes are removed from the body by the nephridium.

• Mostly dioecious.

• Fertilization is both external & internal, oviparous with indirect development.

• Embryo developed into a free-swimming larva called a trochophore.

• Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devilfish), Aplysia, Dentalium (Tusk shell), Chaetopleura (Chiton)

Fig: Octopus (Devilfish)

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