What is the Difference Between Polychaetes and Oligochaetes

The main difference between polychaetes and oligochaetes is that the polychaetes have a pair of parapodia per body segment that bear many bristles. But, oligochaetes have few bristles on their outer surface of the body but, no parapodia. Furthermore, polychaetes include bristle worms that are generally marine, while oligochaetes include earthworms that are aquatic and terrestrial. 

Polychaetes and oligochaetes are two subclasses of phylum Annelida. Generally, annelids are segmented worms that live in marine, aquatic, and terrestrial habitats. 

Key Areas Covered 

1. Polychaetes
     – Taxonomy, Characteristics, Behavior
2. Oligochaetes
     – Taxonomy, Characteristics, Behavior
3. What are the Similarities Between Polychaetes and Oligochaetes
     – Outline of Common Features
4. What is the Difference Between Polychaetes and Oligochaetes
     – Comparison of Key Differences

Key Terms 

Annelids, Chaetes, Oligochaetes, Parapodia, Polychaetes

Polychaetes – Taxonomy, Characteristics, Behavior 

Polychaetes are a subclass of the phylum Annelida that composes bristle worms. Importantly, the main characteristic feature of polychaetes is the presence of many bristles in parapodia. That is; each parapodium contains many bristles or chaetes made up of chitin. The parapodia are fleshy protrusions, which arise in pairs per each body segment. Some common examples of polychaetes include the lugworm (Arenicola marina) and the sandworm or clam worm Alitta.

Figure 1: Bristle Worm Anatomy 

Further, the other characteristic feature of bristle worms is their habitat. Typically, they are marine. Of them, 2% of polychaetes live in freshwater. Also, they can tolerate both the coldest and highest temperatures of the ocean. Besides, bristle worms can grow up to 10 cm in length. Especially, the well-developed head of polychaetes projects forward over the mouth. Also, it contains two or five pairs of eyes, a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia. In addition to these, they can be either brightly-colored, iridescent or luminescent. Regularly, most of the polychaetes crawl along the bottom of the sea while few of them are burrowing, swimming, pelagic life, tube-dwelling or boring.  

Oligochaetes – Taxonomy, Characteristics, Behavior 

Oligochaetes are another subclass of the phylum Annelida that compose earthworms. In comparison to polychaetes, oligochaetes have few bristles on their outer surface of the body with no parapodia. Generally, earthworms are either aquatic or terrestrial, and the terrestrial earthworms burrow into the soil. Besides, they can be either free-living, commensals or parasites. Typically, the largest species of earthworms such as the giant Gippsland earthworm (Megascolides australis) and the Mekong worm (Amynthas mekongianus) can grow up to 2 to 3 meters. 

Figure 2: Earthworm Anatomy

Moreover, the first segment of earthworms is prostomium. It is a smooth lobe with sensory organs and extended tentacles. The rest of the segments only contain bristles but no appendages. Generally, the bristles are longer in aquatic forms. Further, each body segment of an earthworm contains four bundles of chaetes with two on the underside. And, each bundle contains 1-25 chaetes. In contrast to polychaetes, oligochaetes are hermaphrodites. Also, as they mainly live in terrestrial habitats, they lack the external transfer of sperms, and their fertilization occurs in the clitellum or cocoon.  

Similarities Between Polychaetes and Oligochaetes  

• Polychaetes and oligochaetes are two subclasses of the phylum Annelida.  
• Both can be free-living.  
• Also, both have a true coelom derived from the mesoderm.    
• Further, they both have a segmented body with bilateral symmetry.  
• Their body is divided into head, body, and tail.  
• They have an epidermis covered by an acellular, external cuticle.  
• Besides, their circular and longitudinal muscles occur interior to the epidermis.  
• Moreover, sates/chaetes or bristles are the chitinous hairlike extensions anchored into the epidermis in each segment. 
• Further, they have a complete digestive system, a closed circulatory system with dorsal and ventral blood vessels and a well-developed nervous system with a nerve ring of fused ganglia present around the pharynx but, lack a  well-developed respiratory system.    
• A  pair of metanephridia occurs in each segment for excretion.  
• They prefer cross-fertilization. 

Difference Between Polychaetes and Oligochaetes 

Definition 

Polychaetes refer to the marine annelids, with unsegmented swimming appendages with many chaetes. Meanwhile, oligochaetes refer to the hermaphroditic terrestrial or aquatic annelids that lack a specialized head. 

Examples 

Polychaetes include bristle worms, while oligochaetes include earthworms. 

Habitat 

Polychaetes are typically marine, while oligochaetes live in both aquatic and terrestrial habitats. 

Head 

Polychaetes have a well-developed head, while oligochaetes have a less-developed head. 

Parapodia and Chaetes 

Polychaetes have parapodia with many chaetes, while oligochaetes have few chaetes with no parapodia. 

Mode of Nutrition 

Polychaetes are mainly free-living, while oligochaetes can be either free-living, commensals or parasites. 

Type of Reproduction 

Polychaetes are dioecious with temporary or seasonal gonads, while oligochaetes are monoecious with permanent gonads.  

Fertilization 

Polychaetes undergo external sperm transfer and external fertilization. But, oligochaetes undergo no external sperm transfer but their fertilization occurs in the clitellum or cocoon. 

Conclusion 

Polychaetes are a subclass of annelids, having many bristles arranged in parapodia. They include bristle worms that are dioecious with temporary and seasonal gonads. Typically, polychaetes exclusively live in marine habitats. Meanwhile, oligochaetes are another subclass of annelids, having few bristles but no parapodia. They live in both terrestrial and aquatic habitats. In contrast to polychaetes, oligochaetes are earthworms that are monoecious with permanent gonads. Therefore, the main difference between polychaetes and oligochaetes is the presence of bristles and parapodia and the type of reproduction. 

References:

1. “Phylum Annelida | Biology for Majors II.” Lumen, Available Here.
2. Myers, Phil. “Polychaeta.” Animal Diversity Web, Available Here.

Image Courtesy:

1. “Polychaeta anatomy en” By © Hans Hillewaert (CC BY-SA 4.0) via Commons Wikimedia   
2. “Earthworm head” By KDS4444 – Own work (CC BY-SA 4.0) via Commons Wikimedia   

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