Classifying Animals: A Unified Model - Essay Sample

Introduction

Animals differ in complexity, ranging from simple forms like sea sponges and crickets to more complicated types like chimpanzees, and scientists have to classify them with a unified method. Scientists have to isolate characters familiar to all animals as well as those that can differentiate among associated clusters of animals. Animal classification model involves distinguishing animals depending on their anatomy, evolutionary history, morphology, aspects of embryological growth, and genetic makeup (Knott et al., 2018). The classification system continuously advances as new information about species emerges. Comprehending and classifying the different variety of living species makes people understand how to protect the diversity of life on earth. The purpose of this paper entails examining animal diversity by assessing diversity at a lower taxonomic level in the significant and minor phylum.

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Diversity at a Lower Taxonomic Level in a Major Phylum

The Asteroidea represents one of the most important and most common classes within the Phylum Echinodermata. These animals, typically called sea stars or starfishes, represent a varied and speciose group. There exist about 1600 species in existence found across the oceans in the world, from frigid polar waters to the tropics (Linchangco Jr et al., 2017). Besides, there exist about seven orders, namely Brisingida, Notomyotida, Forcipulatida, Spinulosida, Velatida, Paxillosida, and Valvatida. Some of these animals exist as voracious predators having considerable influence on the ecosystem. For example, the crown-of-thorns starfish, Acanthaster planci, can cause severe detrimental impacts to coral reefs, especially during the population epidemic.

Image 1: A Typical Starfish

Image 1 above shows a typical starfish, Asterias rubens, with tube feet on the far-end of the arm. Cushion stars, Culcita novaeguineae, may have short wings that appear as a ball instead of a star. Starfish exist as aquatic invertebrates (Baeta et al., 2016). Similar to other asterozoans, asteroids have a typical star-shaped body feature comprising of a central disc and several (basically five) radiating arms. Asteroids have distinct arm structure that separates them from other asterozoans. The upper or aboral surface usually appear smooth, spiny, or granular and enclosed with overlying plates. Most species have bright colors in different shades, such as orange, red, blue, grey, or brown (Linchangco Jr et al., 2017). Starfish have tube arms driven by a hydraulic system and a mouth at the epicenter of the oral or lower surface. Besides, most species have unique feeding conducts, such as eversion of their bellies and deferral feeding. Some of the species can rejuvenate damaged parts or lost limbs. Besides, they can shed appendages as a way of defense. The Asteroidea have sophisticated life cycles and can reproduce both asexually and sexually.

Image 2: Morphology of Asteroids

Image 2 above shows the aboral and oral surfaces and the transverse section view of a standard arm of a typical asteroid (Knott et al., 2018). The picture also shows the arched ambulacral ossicles forming the dorsal podia pores between ambulacral ossicles.

Image 3: Sea Stars and Starfishes

Image 3 shows various starfishes and their different unique characteristics, such as the number, color, and suckered tube feet (Baeta et al., 2016).

Table 1: Table Showing Orders in Class Asteroidea

Order Species and General Description

Brisingida Contains about 100 species in 17 genera and six families. Examples include Velcro sea star and Brisingid sea star.

Forcipulatida Have 68 genera with about 300 species. Examples include the Northern Pacific sea star, Ocher star, and Sunflower star, Common starfish.

Notomyotida Have about 75 species with 12 genera. Examples include the Benthopecten species.

Paxillosida Contains about 255 species in 46 genera and five families. Examples include the Sand star, Luidia magnifica

Spinulosida Have over 120 species in nine genera and two families. An example is the Echinaster sepositus.

Valvatida Have over 695 species in 172 genera in 17 families. Examples include the Crown-of-thorns, Cushion star, Leather star, Blue starfish, Solaster dawsoni, and Hippasteria spinosa.

Velatida Contain around 200 species in 25 genera and five families. An example include Pteraster capensis.

Table 1 shows the seven orders in class Asteroidea. Brisingida refers to the order in the phylum Echinodermata that exists in deep seas (Knott et al., 2018). Besides, they have between six to 18 long, attenuated arms, which they employ during suspension feeding. Moreover, they have a single series of marginal, a fused ring of disc plates, and lack actinal plates. Furthermore, they have a spook-like ambulacral column, reduced abactinal plates, and crossed pedicellariae. Forcipulatids consist of a small stalk and three skeletal ossicles. Besides, they share with the order brisingida unique pedicellariae (Knott et al., 2018). Forcipilates have more healthy bodies.

Notomyotida exists as deep-sea inhabitants with flexible arms. The inner dorsolateral surface of the appendages has distinctive longitudinal muscle ensembles (Baeta et al., 2016). Moreover, paxillosida remains an enormous order of sea stars. Adults do not have an anus and lack suckers on their tube feet. Besides, they do not grow the brachilaria phase in their early growth.

Paxillosida contains marginal plates, and possess sessile pedicellariae. They mostly live in the soft-bottomed surroundings of sand and mud (Baeta et al., 2016). Moreover, spinulosids entirely do not have pedicellariae but have a unique skeletal structure. Their names originate from the existence of several low spines on the upper (aboral) surface. Scientists have not discovered any fossil spinulosids. The valvatida order consists of both small species that have a small diameter. Most of the species in this order have five appendages with tube feet and suckers.

They have conspicuous marginal ossicles that distinguish most of the species. Some species contain paxillae. However, other species have pedicellariae that appear as clamp-like and buried into the skeletal plates. The velatida order typically includes thick bodies with massive discs. The order encompasses several colds or deep-sea stars with general global distribution. Velatids have a pentagonal or star shape, with arms that range from 5 to 15 (Knott et al., 2018). They have a weakly developed skeleton, which gives them better flexibility, and several papillae on the aboral surface that permits them to respire in limited oxygenated waters. Velatids’ pedicellariae contain spines.

Phylogenetic Tree of Asteroidea

Some scientists have stressed on the significance of ambulacral traits to asteroid categorization and focusing on the difference between Paleozoic and post-Paleozoic types. However, some biologist has emphasized on the absence of suckered tube feet in the paxillosida, terming them primitive (Baeta et al., 2016). From this deduction, two classifications emerged, a basal paxillosida and the rest of the asteroids that have suckered tube feet. These asteroids that have suckered tube feet have the name Surculifera (Baeta et al., 2016). The phylogenic consideration, therefore, groups forcipulatida and brisingida in the superorder of forcipulatacea, while the remaining orders (paxillosida, notomyotida, velatida, spinulosida, and valvatida) in the superorders valavatacea and spinulosacea. Outgroup contrast with calliasterella and addition of the trichasteropsida forms a basal forcipulatacea.

Figure 1: Asteroidea Relationships

Figures 1, 2,3,4,5, and 6 show the relationships between asteroids (Linchangco Jr et al., 2017). The associations and intra-order relationships remain contentious, and research on asteroid phylogeny continues.

Figure 2: Asteroidea Relationships

Figure 3: Asteroidea Relationships

Figure 4: Asteroidea Relationships

Figure 5: Asteroidea Relationships

Figure 6: Asteroidea Relationships

Diversity at a Minor Phylum

Ctenophora (commonly called comb jellies) refers to a phylum of invertebrates that stay in aquatic environments across the globe. They have several cilia that distinguish them and enable them to swim. Examples include cydippids that have egg-shaped bodies and contain a pair of telescopic tentacles fused with tentilla (tiny arms) covered with colloblasts. Besides, they have sticky cells that seize prey. Their bodies include a mass of jelly with a thick layer of two compartments, one on the outside, and the other on the inner lining cavity. The phylum has several body forms, such as the large-mouthed beroids that feed on other ctenophores, the flat platyctenids without combs, and the egg-shaped cydippids that have retractable tentacles they use in catching their prey. There exist two classes in the phylum, namely tentaculata and nuda.

Table 2: Table of Class and Orders of Phylum Ctenophora

Class Orders

Tentaculata Cestida, Cambojiida, Cryptolobiferida, Cydippida, Geneshida, Lobata, Platyctenida, Thalassocalycida.

Nuda Beroidae

Table 2 above indicates the class and orders of phylum ctenophore (Whelan et al., 2017). Tentaculata refers to a type of ctenophore. The distinguishable characteristic of this class entails their pair of long, feathery, retractable tentacles that can contract into specialized covers of cilia. Some species have reduced tentacles with additional secondary limbs. The arms consist of colloblasts. The colloblasts contain sticky-tipped cells that capture small prey. From table 2 above, the class of tentaculata consists of eight orders that include cestida, cambojiida, cryptolobiferida, cydippida, geneshida, lobate, platyctenida, and thalassocalycida (Whelan et al., 2017). Their body size differs significantly. Examples comprise of the tiny, oval sea gooseberries that live on both the Pacific and Atlantic oceans.

Image 4: Ctenophera Diversity

Image 4 above shows various comb jellies, in which A represents Pleurobrachia bachei, B, Mnemiopsis leidyi, C, Beroe gracilis with Pleurobrachia pileus, D, Thalassocalyce inconstans, and E, Coeloplana astericola. Cestids have a flat, ribbon-like form with two short tentacles and two to four reduced cilia combs (Whelan et al., 2017). The unique body shape permits the animals to swim. The animals swim by a method of a muscular furrow, as well as by utilizing their cilia. Cryptolobatidae refers to the only family member in the monotypic order Cryptolobeferida and has two genera, each with one species. Cydippida has a distinguishable oval or spherical bodies with branched, retractable tentacles. The tentacles can contact bags on all sides of the pharynx (Whelan et al., 2017). Genesha refers to the single genus in the monotypic family of Geneshidae with a unique pair of minor lobes around the mouth. Besides, they have extended pharynx.

Phylogenetic Tree of Ctenophora

Scientists argue that ctenophores institute the second-original branching animal ancestry, with sponges as the sister-group to the entire multicellular organisms. Molecular phylogenetics research has shown that the general lineage of the present ctenophores appeared like cydippids, originating from several cydippids after the Cretaceous-Paleogene annihilation incident some million years in the past (Whelan et al., 2017). The neural and muscle cell types either got lost in major animal ancestries (Placozoa and Porifera) or evolved individually in the ctenophore descent. Figure 7 below shows Ctenophora's relationships and the origin of the canonical nervous system of cnidarians and bilaterians (Whelan et al., 2017). Besides, the phylogenetic tree indicates the source of the alternative nervous system in ctenophore and the loss of the nervous system.

Figure 7: Ctenophora Relationsh...