Classification of invertebrate zoology(phylum Annelida)

 

 

Phylum Annelida;

The word annelida is derived from Latin word annelus mean little ring as well French anneler means to arrange in rings.

Characteristics of Phylum Annelida

Phylum Annelida possess a long and segmented body. They are bilaterally symmetrical and triploblastic in nature. They also show the organ system grade of organization that shows the organ differentiation. The body of Annelida is covered, with thin cuticle. The Annelida are having true coelomate in their body. They are mostly live in moist environments such as, in moist soil or freshwater as well in marine water. The annelid exhibit parapodia as well chitinous setae that is used for purpose of locomotion. The body of Annelida is red in color due to the presence of hemoglobin. They are having excretory as well nervous systems. They are having complete digestive system. In Annelida the phenomenon of respiration take place through the body surface. The annelida are having a closed type circulatory system that is consider to be main transporting system of the body. The Annelida exhibit with separate, united sexes therefore they are known as hermaphrodites. In annelida the mechanism of fertilization may be internal or external.

Classification of phylum annelida;

The phylum annelida is differentiated into four classes.

Class 1; Polychaeta (gr; poly mean many & chaeta mean hair/bristles;

The member of Polychaeta are chiefly marine while some of them live in freshwater. They are carnivore in nature. Their body is segmented internally and externally. They are having head with prostomium and peristomium that bears eyes or tentacles cirri and palps. They are having numerous setae as well lateral outgrowth known as parapodia. They don’t have clitellum within body. They take respiration through cirri or branchiae. They are having spacious coelom ,generally divided by intersegmental septa.The member of Polychaeta exhibit alimentary canal along-with eversible buccal region as well potrusible pharynx. The excretion occur in these invertebrates through the paired organ known as nephridia. They are having separate sexes, while gonads are temporary in many segments. They exhibit external fertilization, the asexual reproduction occur through budding. The trochophore larva is present in these invertebrates.

The class Polychaeta is differentiated into four orders;

Order 1. Errantida                   

Order 2. Archianellida

Order 3. Sedentarida

Order 4. Myzostomarida

 

Class 2 Oligochaeta (Gr; Oligos mean few & Chaeta mean hair)

They are mostly terrestrial, while some live in freshwater. They are conspicuous body with external as well internal segmentation. They are having indistinct head with no sensory organs. They are having few setae which is embedded in the skin. They don’t have any para-podia with body. They exhibit glandular clitellum that is responsible for cocoon formation. They are having pharynx without eversible and jaws. They are hermaphrodite in nature, for instance united sexes. They are having testes that is located anterior to ovaries. The direct development occur in this class, while fertilization is external without larval stage.

This oligochaeta is further divided into three orders;

Order 1. Plesiopora

Order 2. Prosopora

Order 3. Opisthopora

 

Class 3. Hirudinea (L; hirudo mean leech)

The member of this hirudinea are mostly blood sucking ectoparasites, or carnivorous. They are marine, terrestrial or freshwater in habitat. Their body is usually elongated, flattened as well dorsoventrally or cylindrical. Their body comprises of a fixed number of segments for instance (33), and each segment divided into 2 to 4 rings or annuli. They exhibit external segmentation external with no internal septa.In these invertebrates’ par-podia and setae are absent. In the hirudinea body the ventral suckers are present ventrally at both anterior and posterior ends. Their mouth opens into anterior suckers on the ventral surface, however anus opens into posterior suckers on the dorsal surface. The coelom are absent because of filling by botryoidal tissues, that is form haemocoelomic sinuses. They are hermaphrodite in nature, with one male as well one female gonopore. In hirudinea fertilization is internal. The asexual reproduction is absent in hirudinea. Their eggs are always laid in the cocoons. In hirudinea direct development take place without the free swimming larval stage.

This class is differentiated into four orders;

Order 1. Rhynchobdellida

Order 2. Branchiobdellida

Order 3. Acanthobdellida

Order 4. Gnathobdellae

 

Class 4. Archiannellida (gr; arch; first)

They are exclusively marine in nature. They are having elongated and worm like body. In archiannellida the setae and para-podia are not present. In these invertebrates the external segmentation is usually marked by faint but internal segmentation is marked by coelomic-septa.They are having prostomium with 2 or 3 tentacles. They are usually hermaphrodite with separate sexes. The trochophore larva is present in archiannellida. The Dinophilus, polygordius and protodrilus belong to this class.