Exam: Animal Diversity - PDF

# Tema 2. DIVERSIDAD ANIMAL 1. INVERTEBRADOS BASALES: PORÍFEROS. PLACOZOOS, CNIDARIOS Y CTENÓFOROS ## ORIGEN DE LOS METAZOOS - Teoría colonial (Haeckel, 1874): A colony of flagellated protists gave rise to an ancestral "Planuloid" (the planula is the larval form of Cnidarians) which later underwe...

# Tema 2. DIVERSIDAD ANIMAL 1. INVERTEBRADOS BASALES: PORÍFEROS. PLACOZOOS, CNIDARIOS Y CTENÓFOROS ## ORIGEN DE LOS METAZOOS - Teoría colonial (Haeckel, 1874): A colony of flagellated protists gave rise to an ancestral "Planuloid" (the planula is the larval form of Cnidarians) which later underwent invagination and gave rise to a "Gastrea" with a blastopore resembling the gastrula of many current animals. - Teoría de la "Troquea" (Nielsen, 2018): This theory proposes the origin of the ancestral Protostome, with a trochophore larva and a ventral nerve cord. ## Phylum Poríferos (Porifera) - `Sésiles` adults, shaped like a tube, with one end closed. - Consists of three parts: - Epidermis: formed by cells called pinacocytes (lacking a basal membrane). - Espongiocele or Atrium: lined with choanocytes, cells with a collar that surrounds a cilia (organic particles are captured by the collar ingested by pseudopods, then ingested by amoebocytes and transported to the entire body). - Mesoglea or Mesohilo: consists of several cell types embedded in a collagen matrix secreted by spongiocytes and collenocytes. - Other cell types: - Porocytes: cells with a central channel that regulates the water supply. - Myocytes: contractile cells with slow response, arranged in the mesoglea and around the osculum. - Amoebocytes: cells of the mesoglea capable of transforming into any other cell type. ## Simetria animal 1. **Simetría radiada:** This symmetry is typical of Cnidarians and Echinoderms, although in the latter, the larva has a bilateral structure. In this symmetry, the body can be divided into two similar halves along an axis through the central axis. It is typical of sessile animals that have plankton feeding and it is available in any direction, therefore, the organs responsible for capturing food are arranged radially and animals do not have to move to feed. 2. **Simetría bilateral:** This is the symmetry of most current animals and is associated with movement in a defined direction, usually "forward", in cephalized animals, with a well-defined anterior nervous system and a wide variety of sensory organs arranged anteriorly. This symmetry defines an anteroposterior axis (generally mouth-anus), and another dorsoventral axis. ## Animals "Neuralia" The main feature of the animals that make up this group (Cnidarians + Ctenophores + Bilaterals) is the presence of specialized cells in communication: **Neurons**, propagation of signals through action potentials using Na*/K* channels, electrical synapses (through gap junctions) and chemical (through neurotransmitters). ## Phylum Placozoos (Placozoa) - Small metazoans (1-3 mm), flat. - Formed by two layers (upper and lower) made up of hundreds of ciliated cells with an undifferentiated mesenchyme between them. - Adults are discoid and asymmetrical. - Have six types of somatic cells - Intercellular junctions are desmosome-type. - Have structures called "bright spheres" which possibly have a defensive function. - Lack digestive systems. ## Filogenia de los Metazoos The most represented evolutionary hypothesis in phylogenies in the last two decades proposes that the Phylum Porifera is the basal group of Metazoans and that Placozoans are the sister group of Eumetazoans (Cnidarians, Ctenophores and triblasts). The basal position of Porifera has been based on the fact that sponges lack: 1. Embryonic ectoderm and endoderm homologous to those of other animals. 2. True intercellular junctions. 3. The ability to make behavioral responses in line with the absence of muscle cells (effectors), sensory and nervous cells. Placozoans also lack muscle and nerve cells, as well as a basal membrane in their epithelia, but unlike sponges, they do have junctions between the cells of the outer epithelia. ## Phylum Cnidarios (Cnidaria) ### Characteristics of Cnidarians 1. Diblastic Metazoans, with ectoderm and endoderm separated by a Mesoglea derived from the ectoderm, acellular or with abundant collagen and cells. 2. Two morphological types: `sessile` (polyps, asexual) and `mobile` (medusa, sexual). 3. Primarily radial symmetry, often modified as biradial or quadradial and an `oral-aboral axis` (mouth-anus). 4. They are active predators and they have specialized cells for capturing food (`Cnidoblastos`, with `Nematocistos` inside them). 5. Musculature formed by `epitheliomuscular` cells. ### Sistemática de Cnidarios 1. **Subphylum Antozoos.** Marine. Solitary Individuals or colonial, without medusa shape. They reproduce sexually and `asexually`. With `Cnidoblastos` in a divided gastrovascular cavity. 2. **Subphylum Endocnidozoos.** Marine, brackish water and freshwater. They are intracellular parasites of `Vertebrates`, `Annelids` and `Brachiopods`. 3. **Subphylum Medusozoos.** Marine and freshwater. Free-living and sessile forms. The body is typically tetraradial. ## Phylum Ctenóforos (Ctenophora) ### Characteristics of Ctenophores 1. Diblastic Metazoans. Ectoderm and endoderm separated by a cellular mesenchyme. Biradial symmetry. 2. With adhesive structures on the tentacles called `Coloblastos`. 3. Unique gastrovascular cavity. No respiratory, circulatory or excretory systems. Striated musculature. Nervous system in the form of a nerve network. 4. Eight rows of ciliated plates called `Ctenas`. Some adults and many juveniles with a pair of long tentacles. 5. The majority are hermaphrodites. The larval stage is called `cidipoid larva`.