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The archenteron initially forms, and the mesoderm splits into two layers: the first attaches to the body wall or ectoderm, forming the parietal layer and the second surrounds the endoderm or alimentary canal forming the visceral layer. The space between the parietal layer and the visceral layer is known as the coelom or body cavity.
The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle. The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal muscle cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton ...
Ascaris is a nematode genus of parasitic worms known as the "small intestinal roundworms". [1] One species, Ascaris lumbricoides , affects humans and causes the disease ascariasis . Another species, Ascaris suum , typically infects pigs .
Trichinella is the genus of parasitic roundworms of the phylum Nematoda that cause trichinosis (also known as trichinellosis). Members of this genus are often called trichinella or trichina worms. A characteristic of Nematoda is the one-way digestive tract, with a pseudocoelom (body cavity made up of only an ectoderm and endoderm).
Like other bilaterians, they have three main cell layers (endoderm, mesoderm, and ectoderm), [5] while the radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. [6] Beyond that, they are "defined more by what they do not have than by any particular series of specializations."
Ascaris lumbricoides is a large parasitic roundworm of the genus Ascaris. It is the most common parasitic worm in humans. [1] An estimated 807 million–1.2 billion people are infected with Ascaris lumbricoides worldwide. [2]
Below this are two layers of muscles, which develop from the lining of the coelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips, [23] whose contractions make the segment shorter and fatter. [8]
The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to ca. 73,000 deuterostome species. [6] Protostomes are divided into the Ecdysozoa (e.g. arthropods, nematodes) and the Spiralia (e.g. molluscs, annelids, platyhelminths, and rotifers).