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Protostomia classification and characteristics

Protostomia classification and characteristics


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While working through a practice test for my final exam in an introductory organism biology course, I came across this question:

Protostome characteristics include which of the following?
a) a mouth that develops secondarily, and far away from the blastopore
b) coelom formed by schizocoely
c) radial body symmetry
d) spiral cleavage
e) none of the above

After reading the answers a few times over, I just moved on because I couldn't figure out how to answer. My first instinct said that both (b) and (d) were correct, but the answer is (e).

After some thought, I've come to a few ideas how this could be true.
1. The wording is incorrect, the definition he provides for protostomia says "body cavity formed by schizocoely," and "spiral determinate cleavage."
2. These two characteristics are not present in all protostomes (i.e. spiralia evolves after ecdysozoa, and platyzoa have no body cavity).

Just looking for some clarity or closure regarding this.


1.1) Characteristics of living organisms

Movement: is an action by an organism or part of an organism causing a change of position or place.

Respiration: describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Sensitivity: is the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses.

Growth: is a permanent increase in size and dry mass by an increase in cell number or cell size or both.

Reproduction: is the processes that make more of the same kind of organism.

Excretion: is the removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials and substances in excess of requirements.

Nutrition: is the taking in of materials for energy, growth and development. Plant require light, CO2, H2O and ions. Animals need organic compounds and ions and H2O.


Protostomia classification and characteristics - Biology

Figure 1. The Caribbean reef squid (Sepioteuthis sepioidea) is a complex lophotrochozoan.

Animals belonging to superphylum Lophotrochozoa are protostomes, in which the blastopore, or the point of involution of the ectoderm or outer germ layer, becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band and then splits internally to form the coelom this protostomic coelom is hence termed schizocoelom.

As lophotrochozoans, the organisms in this superphylum possess either a lophophore or trochophore larvae. The lophophores include groups that are united by the presence of the lophophore, a set of ciliated tentacles surrounding the mouth. Lophophorata include the flatworms and several other phyla. These clades are upheld when RNA sequences are compared. Trochophore larvae are characterized by two bands of cilia around the body.

The lophotrochozoans are triploblastic and possess an embryonic mesoderm sandwiched between the ectoderm and endoderm found in the diploblastic cnidarians. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are symmetrical. It also means the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.


Cestodes: Classification and General Characteristics

Cestodes or tapeworms are the members of the class Cestoda of the phylum Platyhelminthes. About 6000 species of cestodes are identified as parasites and they are mainly intestinal. They are both marine and terrestrial vertebrates are the definitive hosts and their intermediate hosts are crustaceans, insects, annelids, mollusks, etc.

The species T. saginata (beef tapeworm) can grow up to 65 ft (20m) in length while the largest tapeworm, known as the whale tapeworm (Tetragonoporus calyptocephalus) can grow up to 100 ft (30 m) or more. Besides these, the smallest parasite, known as vole and lemming tapeworms, can grow up to only 13–240 mm in length.

Taenia solium: Image credit-Wikimedia Commons

Among the cestode species, Taenia solium, Taenia saginatum, Hymenolepis nana and Diphyllobothrium latum are the most common parasite of human.

Classification of Cestodes

According to Habitat, the cestodes are the following types:

A. Intestinal Tapeworms: Adults live in intestine of human.

  1. Taenia saginata (Beef tapeworm)
  2. Taenia solium (Pork tapeworm)
  3. Hymenolepis nana (Dwarf tapeworm)
  4. Hymenolepis diminuta ( Rat tapeworm)
  5. Diphylobothrium latum (Fish tapeworm)
  6. Diphylidium carinum (Double-pored Dog tapeworm)

B. Tissue Tapeworms-Larval Stage

  1. Hydatid cyst of Echinococcus granulosus (Dog tapeworm), E. multilocularis
  2. Cysticercus cellulose of Taenia solium.
  3. Coenurus cerebralis of Multiceps multiceps.
  4. Plerocercoid of Sparganum mansoni and Sparganum proliferum.

General Characteristics of Cestodes

1. Adult Cestodes: Adult cestodes have tape-like, ribbon shaped and segmented body and the length varies from few mm to several meters. They are flattened dorso-ventrally. An adult worm has three regions:

Scolex of Taenia solium: Image credit-Wikimedia Commons

Head is provided with attachments. Rostellum is a beak-like projection on the head which carries hooklets.

2. Proglottid or Segnments: Each proglottid or segment is essentially a functional individual, i.e. a complete unit of a tapeworm. A segment is called immature if the male and female reproductive organs are not differentiated, mature if the reproductive organs are differentiated and gravid if the uterus is filled with eggs. These are seen from front to backwards.

Progloyid of Taenia solium: Image credit: wikimedia commons

3. Mouth, alimentary and body cavity are absent. Glucose or other simple nutrients are absorbed directly from the host gut.

4. Excretory and nervous systems are present.

5. Reproductive system: Each worm is a hermaphrodite, i.e. sexes are separate.

(a) Male genital system: It lies on the dorsal surface of each segment and mature before the female genital system. It consists of testes, vas deferens and cirus(penis). The cirus and vagina open into a common genital pore which is on the mid-ventral surface in Pseudollidea and on the lateral border in Cyclophylidea.

(b) Female genital system: It lies on the ventral surface and consists of ovary (single or paired), vagina from genital pore to ootype, uterus which is open in Pseudophyllidea and a blind sac in Cyclophyllidea and Ootype where ovum is fertilized.

6. Fertilization: It takes place between the segments. It may be a self-fertilization or a cross-fertilization between the segments of same or other worm.

7. Eggs: Eggs are formed in ootype and are present in large numbers in gravid segments. An egg may be:

(a) Operculated has a ciliated epithelium or

(b) Non-operculated in Cyclophyllidea. It has two coverings-outer egg-shell and inner embryophore which surrounds the embryo. The formed embryo is a six hooked (hexacanth) sphere called oncosphere or hexacanth embryo. The space between the embryophore and the eggs shell contains yolk material.

8. Larva: The following types of larvae are found in cestodes:

(a) Cysticercus: The entire larva is transformed into a bladder from which the head or scolex of the worm sprouts. One adult worm develops from each scolex. This consists of a bladder with one scolex as in Taenia saginata and Taenia solium.

(b) Hydatid cyst: This is the cystic larval form. A bladder multiplies through the process of budding and produces many bladders. On the wall of these cysts, brood capsules are produced, inside which the scolices as in Enterococcus granulosus.

(c) Cysticercoid: The entire larva is solid containing a scolex, e.g. in Hymenolepis nana.

(d) Plerocercoid and procercoid are found in fish tapeworm.

Economic Importance of Cestodes

The most of the tapeworms are the harmful organisms. They are the parasitic worms and they are found in the intestine of human(adult form) and cattle(larval forms). Both the tapeworms and bladderworms contribute to loss of meat production of cattle throughout the world. Besides these, they have a few economic importances. In some cases, the tapeworms act as natural fertilizing agent and help to increase the fertility of the soil.


Protozoa: Definition, Characteristics, Classification and Types

Protozoa are eukaryotic, unicellular microorganisms, which lack cell wall.

Characteristics of Protozoa:

The major distinguishing characteristics of protozoa are given below:

1. They do not have cell wall some however, possess a flexible layer, a pellicle, or a rigid shell of inorganic materials outside the cell membrane.

2. They have the ability during their entire life cycle or part of it to move by locomotor organelles or by a gliding mechanism.

3. They have heterotrophic mode of nutrition, whereby the free-living forms ingest particulates, such as bacteria, yeast and algae, while the parasitic forms derive nutrients from the body fluids of their hosts.

4. They reproduce primarily by asexual means, although in some groups sexual modes also occur.

Classification of Protozoa:

The classification of protozoa is mainly based on their means of locomotion. They are subdivided into the following four classes (or subphyla by some taxonomists). Species marked with asterisks (*) have been described in details with illustrations.

Motility is due to the streaming of ectoplasm, producing protoplasmic projections called pseudopodia (false feet). Examples: Free-living form like Amoeba proteus* and parasitic form like Entamoeba histolytica*.

Locomotion is effected by one or more whip-like, thin structures called flagella. Examples: Free- living forms like Euglena viridis*, Cercomonas longicauda*, Heteronema acus* and parasitic forms like Trichomonas vaginalis, Trypanosoma gambiense*, Giardia lamblia*.

Locomotion is carried out by means of short hair-like projections called cilia, whose synchronous beating propels the organisms. Examples: Free-living forms like Paramecium caudatum*, Stentor polymorpha*, Vorticella campanula* and parasitic form like Balantidium coli*.

Unlike the above three classless of protozoa, members of the class sporozoa do not have locomotor organelles in their mature stage however, immature forms exhibit some type of movement. All the members of this group are parasites.

Examples: Plasmodium, the malarial parasites of animals and human beings.

Types of Protozoa:

Based on the mode of nutrition, protozoa are of the following two types:

1. Free-living protozoa: They ingest particulates, such as bacteria, yeast and algae.

2. Parasitic protozoa: They derive nutrients from the body fluids of their hosts.


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Frequently Asked Questions( Five Kingdom Classification System)

What is classification?

Classification is the arrangement of plants and animals in taxonomic groups according to the similarities and differences observed.

What is kingdom classification?

Kingdom classification is the highest classification into which the organisms are grouped in the taxonomy. It is ranked above the phylum.

The two kingdom classification was proposed by Carolus Linnaeus. He classified the living organisms on the basis of nutrition and mobility. The living organisms were classified into Kingdom Plantae and Kingdom Animalia.

The living organisms are divided into 5 different kingdoms – Protista, Fungi, Plantae, Animalia, and Monera on the basis of their characteristics such as cell structure, mode of nutrition, mode of reproduction and body organization.

Species are the basic unit of classification. The organisms that have the same characteristics and can breed with each other to produce fertile offsprings are known to belong to the same species.

In the two-kingdom classification, the plants included photosynthetic and non-photosynthetic species. Fungi, which feed on dead organic matter, were placed under photosynthetic plants. Therefore, there arose a need for another system of classification where the organisms with the same characteristics were clubbed into one kingdom.

The organisms are classified according to the following different levels- Kingdom, Phylum, Class, Order, Family, Genus and Species.

The prokaryotes are classified into kingdom Monera. There are two other kingdoms including prokaryotes- Eubacteria and Archaea.


Characteristics of Class Cestoda

A complex life cycle - The body of true tapeworms consists of many segments known as proglottids. Each of the proglottids contains both a male and female reproductive structures (as hermaphrodites) that are capable of reproducing independently.

Given that a single tapeworm can produce as many as a thousand proglottids this allows tapeworms to continue thriving. For instance, a single proglottid is capable of producing thousands of eggs, their lifecycle can continue in another host when the eggs are ingested.

Here, the host that ingests the eggs is known as the intermediate host given that it is in this particular host that the eggs hatch to produce a larvae (coracidium). The larvae, however, continues to develop in the second host (definitive host) and mature in the adult stage.

They lack a digestive system - Compared to the two other classes of flatworms, tapeworms lack a digestive system. Instead, the surface of their bodies are covered by small microvillus-like projections similar to those found in the small intestine of many vertebrates.

Through these structures, tapeworms effectively absorb nutrients through their outer covering (tegument). For this reason, a majority of tapeworms can be found in the small intestine of many of their hosts where they can easily obtain nutrition.

They have well-developed muscle.

Modified cilia on their surface are used as sensory endings.

The nervous system is made up of a pair of lateral nerve cords.

Examples of cestodes species include:

  • Taenia solium
  • Dipylidium caninum
  • Taenia saginata
  • Hymenolepis nana
  • Diphyllobothrium latum

The Deuterostomes

In addition to the features listed above, the deuterostomes have (or had) gill slits. (The echinoderms have lost the gill slits of their ancestors.)

Echinoderms (Phylum Echinodermata)

Figure 19.1.10.11 Starfish
  • radial symmetry. HOWEVER, their larvae have bilateral symmetry so the echinoderms probably evolved from bilaterally symmetrical ancestors and properly belong in the Bilateria.
  • water vascular system. Seawater is taken into a system of canals and is used to extend the many tube feet. These have suckers on their tips and aid the animal in attaching itself to solid surfaces.
  • no gill slits
  • About 6,000 species &mdash all of them marine.

There are 5 classes of echinoderms:

  • Sea lilies (Crinoidea)
  • Sea Stars (aka "Starfish") (Asteroidea) The photo (courtesy of Dr. Charles Walcott) shows a sea star that lost an arm and is in the process of regenerating a replacement.
  • Brittle stars (Ophiuroidea)
  • Sea cucumbers (Holothuroidea)
  • Sea urchins and sand dollars (Echinoidea)

For Sporozoans, the sexual process of reproduction involves the formation of opposite sex gametes that may be structurally similar or different. For instance, whereas gametes produced by Coccidiomorpha tend to be structurally different, those produced by Gregarinina are generally similar.

Here, it's also worth noting that for different organisms, different processes are involved in the formation and production of gametes. In Gregarinina, the division of gamont results in the production of gametes. However, in Coccidiomorpha, macrogametes produce the growing gamonts (rather than during division) during microgametogenesis.

The gametes produced by gametocytes then unite to form a zygote. While it's possible to distinguish the sex gametes by size and shape in some species, this is not possible in others. To form the zygote, the microgamete penetrates the macrogamete for fertilization.

The zygote formed through fertilization of the female gamete goes through sporogony to form sporozoites within the oocysts. This may involve a simple division of the zygote or multiple divisions depending on the organism. The sporozoites formed during this phase are the infective forms that penetrate host cells and cause disease.

Within the host cells, the sporozoites (which are known as trophozoites at this stage) can continue reproducing asexually through a process known as schizogony. This involves division of the nucleus to form a multinuclear schizont that undergoes segmentation to form individuals known as merozoites. Some of these individuals penetrate new cells where asexual reproduction continues while others develop to form gamonts involved in sexual reproduction.

* During the metagamic period of their life cycle, Sporozoa can also form spores that are characterized by a protective covering. This form of the parasite allows them to survive unfavorable environmental conditions for a long period of time.


Watch the video: Classification (January 2023).