How many species of Nemertea are there?
Approximately 1300 valid, described species of the phylum Nemertea, or ribbonworms, are known worldwide, inhabiting oceans, freshwaters, and land. Current field-work suggests that at least several times this number remain to be named or discovered.
How are the two classes of Nemertea different?
The protonephridia of nemertea and flatworms are different in structure, and in position – the flame cells of nemertea are usually in the walls of the fluid vessels and are served by “drains” from which the wastes exit by a small number of tubes through the skin, while the flame cells of flatworms are scattered …
Where can you find Nemertea?
Ribbon worms, or nemerteans, are a group of very unusual worms related to mollusks and annelids. Most ribbon worms live in the ocean, and some can grow to over 100 feet long, but are usually much less than an inch wide. Some species live on land or in freshwater. Unlike annelids, ribbon worms are not segmented.
Do Nemertea have a coelom?
Nemertea and Platyhelminthes have traditionally been grouped together because they possess a so-called acoelomate organization, but lateral vessels and rhynchocoel of nemerteans have been regarded as coelomic cavities.
What are characteristics of Nemertea?
Characteristics of Nemertea: Bilaterally symmetrical and dorsoventrally flattened. Body has more than two layers of cells with tissues and organs. Body has a through gut with a mouth and anus. Body has no body cavity.
Is Nemertea segmented?
The species is referred to as ‘segmented’ in zoological textbooks and thus enigmatic from a phylogenetic point of view (e.g. Brusca & Brusca 2003). Its segmentation has also been discussed in the context of whether the last common ancestor of bilaterian animals was segmented (Balavoine & Adouette 2003).
How does phylum Nemertea reproduce?
Most Nemertea reproduce by sexual reproduction, being either male or female. However some freshwater and terrestrial species are hermaphroditic and can be self fertilising. In a very few species, the eggs are retained within the female and hatch as small copies of the adult to then be released into the world.
Which one is the most distinguishing feature of phylum Nemertea?
Nemertea are soft and unsegmented animals (Figure 1). A unique characteristic of this phylum is the presence of a proboscis enclosed in a rhynchocoel. The proboscis serves to capture food and may be ornamented with barbs in some species.
Do Nemertea have a segmentation?
Taxonomic level: phylum Nemertea; grade of construction: organs derived from three tissue layers; symmetry: bilateral; type of gut: complete, with anus; type of body cavity other than gut: rhynchocoel surrounding proboscis; segmentation: absent; circulatory system: present; nervous system: small bundles of nerves ( …
What is the phylum Nemertea?
Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms. Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela.
What is the phylogenetic relationship between Nemertea and proboscis?
Phylogenetic Relationship. 1. Body of Phylum Nemertea is mostly slender; elongated and often flattened, hence called ‘ribbon worms’ or ‘proboscis worms’ (Fig. 14.31). 2.
What is the phylogenetic relationship of Nemertea with turbellarian flatworms?
The phylogenetic relationship of Nemertea with the turbellarian flatworms indicate in several respects such as: (iv) Presence of protonephridia and it is assumed that the Nemertea has evolved from acoelomate ancestor shared with flatworms.
What is the anatomy of a nemertean?
Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species. Inside the concentric tubes of these layers is mesenchyme, a kind of connective tissue. In pelagic species this tissue is gelatinous and buoyant. The mouth is ventral and a little behind the front of the body.