- Body is fish-like and is useful for burrowing and swimming.
- It has a head.
- It shows a tail.
- Appendages are absent.
- Dorsal, caudal and ventral fins are present.
- Body-wall shows one- cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle and parietal peritoneum.
- It has no exoskeleton.
- Notochord extends from the anterior end to posterior end.
- Enterocoelic coelom is present. However, reduced in the pharyngeal region by atrium.
- Alimentary canal is long. It includes a large pharynx with many gill-slits ciliary mode of feeding is developed.
- Gillss will perform respiration.
- Circulatory system is closed.
- Heart and respiratory pigments are absent.
- Hepatic portal system is present.
- Excretory system shows paired protonephridia with solenocytes.
- Brain is not present
- Two pairs of cerebral and several pairs of spinal nerves are present.
- Sexes are separate. Gonads are metamerically arranged and with out gonoducts.
- Asexual reproduction will not take place.
- Fertilization is external.
- Cephalochordata Classification:
- The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single family, Branchiostomidae.The family contains only two genera Branchiostoma and Asymmetron
- SUB-PHYLUM: UROCHORDATA
- SUB-PHYLUM: CEPHALOCHORDATA
- SUB-PHYLUM: VERTEBRATA
In marsupials the embryo develops incompletely in the uterus. They show yolk sac placenta and primitive allantoic placenta: Yolk sac placenta:
Primitve Allantoic Placenta:
In paramoles simple allantoic placenta is developed Allantoic will enlarge It comes In contact with chonon This structure will be closely applied to mother’s uterus. It is called chorio-allantoic placenta. In these animals yolk sac placenta is not seen.
Placenta In Eutherla:
In Eutherian mammals true allantoic placenta is seen. Allantoic becomes big and comes in contact with chorion. This part will show close association with uterine wall. This connection is called placental connection. The structure of placenta will vary in different orders of Eutheria.
Placenta is classified in three ways.
- The placenta classification on nature of contact.
- Placenta is classified basis on the distribution of villi.
- Classification of placenta basing on histology.
Indeciduate type placenta: Ex: Ungulate, Cetaceans, Sirenians. Lemurs:
The chorianic villi are simple projections, they lie in contact with uterus. They have a loose contact. There is no fusion. At the time of birth of embryo uterus is not damaged.
Deciduate type Placenta: Ex: Primates, Rodentia, Insectivora, chiroptera The allantochorianic villi penetrate into uterine valli. They are intimately fused. Hence at the time of birth, the uterus is damaged. Bleeding occurs, the utrine wall enters into formation of placenta is called deciduas.
BASING ON DISTRIBUTION OF VILLI:
According to the distribution of villi five kinds of placenta are seen.
- Diffused type placenta: Ex: Horse, pig. The villi are uniformly distributed on the surface of blastocyst, except at the extreme ends.
- Cotyledonary placenta: Ex: Sheep, Cow, Deer. The villi are arranged in groups. Each group is called cotyledon. Each cotyledon fits into caruncla fo uterus.
- Intermediate type Placenta: Ex: Cainel, Giraffe. It is a rare type, it shows free villi on cotyledons. Hence it is called intermediate type placenta.
In these three types of placenta during perturition the foetus will not damage uterus.
- Zonary placenta: Ex: Cat, Dog, Carnivores. The villi are In the form of transverse zones. In dog a single girdle of vhf will be present. In fox two girdles of villi are present. The villi penetrate Into uterine wall. Hence during parturitlon uterine wall Is damaged.
- Discoidal Placenta: Ex: Rat, Bat, Rabbit. On the entire surface of blastocyst the villi are in the form of discs. When the embryo Is growing It movesaway from uterus hence the with look like a disc. These villi are Intimately connected with uterus. Hence during parturitlon much uterine tissue is damaged.
a) Epithelio chorlal placenta: Ex: Pig, Horse, (Ungulates Lemmures)
- Endothelium of mother blood vessel.
- Maternal syndesmose connective tissue.
- Epitheliurn of mother
- Chorion of foetus.
- Foetus connective tissue (syndesmose
- Endothellum of foetal blood vessel.
If all the six layers are present the placenta is called epithello chorlal placenta.
b) Syndeumose chorial placenta: Ex: Sheep, Cow.
The allanto-chorianic vith will pierce into the uterus of the mother, the chorion will come in contact with syndesmose of mother’s uterus. Hence iti s called syndesmose chorial.
c) Endothelio chorial placenta: Ex: Dog, Carnivores. The chàrion of the foetus will come in contact with the endotheli of mother ‘s uterus, hence it is called endothelio-chorial placenta.
<p><strong>Please include attribution to bioscience.pk with this graphic.</strong><br /><br /><a href='https://www.bioscience.pk/topics/zoology/item/732-placenta-in-mammals-structure-types-and-functions'><img src='https://www.bioscience.pk/images/Types.of.Placenta.in.Mammals.jpg' alt='placentainmammals' 540px border='0' /></a></p>
- Placenta will form a physiological barrier between mother and foetus. It will possess foetal and maternal blood mixing.
- Placenta allows the diffusion of monosacharides, amino adds, hormones, vitamins, oxygen, .carbondioxide, water and other waste materials, because of this it supplies food, oxygen to foetus.
- It works as an excretory organ of foetus. It releases the nitrogenous waste materials Into mother blood.
- It works as an endocrine gland. It will secretes lactogen ,progesterone,etc. hormones.
- The placenta will manufacture fructose from glucose.
- Monocuspici teeth - Teeth with one cusp or tubercie
- Bicuspid teeth - Teeth with two cusps.
- Tricuspid teeth - Teeth with three cusps and all the cusps are arranged in one plane by forming a triangle.
- Bunodont teeth - Teeth with a number of distinct blunt pointed cusps - Man and Monkey.
- Lophodont teeth - The cusps are fused in the form of ridges- Horse & Elephant.
- Selenodont teeth - Here the cusps are wear away, exposing the under lying dentineythich-getnuii by crescent -Shaped bands raf -
- Secodont teeth - The cusps are having sharp cuffing surfaces - carnivorous mammals. Carnassial teeth are present.
- Hypsodont teeth - The teeth are having long crown and deeply placed neck. - Horse
- Brachyodont teeth - Teeth with short crow, and neck at the surface gum. - Dog.
In the beginning the buccal epithelitim of the gum sinks down forming the t ridge. Simultaneously, the cells of malpighian layer of epidermis grow and move into the epidermis forming the dental lamina. Along the dental Lamina, the mesenchymal cells multiply and condense into a dental papilla or tooth bud. This grows upwards and pushes the dental kA Thereby an inverted cup form assumed The malpighian cells become markedly columnar and are known as ameloblasts. The mesenchymal ce are differentiated into a layer of odontoblast cells.
The tooth bud is now as ‘enamel organ’. The ameloblast cells secrete enamel’ towards their inner side, while the odontobiasts secrete ‘dentine’. The enamel sur rounds the upper part of dentine like a cap. The remaining mesenchyme cells of the dental papilla form the ‘pulp’ and the cavity of the enamel organ modifies into the pulp cavity. Sooner, the development of the tooth is completed,. The jaw bone forms a socket or alveolus around its base. The tooth increases in size by the secretion of more or mote dentine and finally emerges out from the gum. The odontoblasts now secrete cement around the neck and root of the tooth narrowing the pulp cavity. In majority of the mammals, the odontoblasts stop functioning after the development of tooth and the tooth stops growth. In some cases, like incisors; of rodents and tusks of elephants, the odontobiasts are functional throughout the life and the teeth continue to grow.
- CONCRESCENCE THEORY OF ROSE
- TRLTUBERCULAR THEORY OF COPE & OSBORN
According to the concrescence theory, the molar tooth has evolved by the fusion of several simple cone-like teeth. This theory Is now abandoned in favor of the second with is now accepted with certain modifications.
Cope & Osborn theory is based on the paleontological evidence. As such the mammalian molar has been derived from a simple reptilian cone by the development of additional cusps. The Mesozoic mammal, the crown of the molar tooth has three cusps arranged more or less in the form of a triangle. This pattern of tooth has been termed tritubercular. It is assumed that the molars of the modem mammals have originated from the primitive tooth by the development of lateral cusps.
In the upper molars the three cusps are known as paracone, protocone, and metacone, The protocone lies on the apex at the inner side, the paracone lies external to it and the metacone to the back of the paracone. In the lower molars the cusps are named protoconid, paraconid and metaconid. The protoconid and metaconid are anterior and posterior to it in position. These cusps are joined by the ridges and later bell-like extensions or talons are produced from the ridges and additional cusps form on them. In this way, the various types of mammalian molars have evolved.
- Root: It is the basal part embedded in the bony socket.
- Neck: it is the part above the root enclosed by the gum
- Crown It is the upper part beyond the surface of the gum.
- Pulp cavity: The entire tooth encloses a central pulp cavity surrounded by a layer of odontoblast cells filled with soft pulp. It is made up of connective tissue, blood vessels and nerve fibers.
- Dentin: A substance chemically similar to bone- dentine forms the major part of the tooth. But the dentine is permeated by numerous thin canaliculi.
- Enamel: It is present over the de in the crown and neck regions of the tooth. It is hardest and contains only traces of living matter.
- Cement: It surrounds the denting of the root portion of the tooth. It is bony in nature.