By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal?s outer covering and the outer lining of the gut cavity, where internal organs develop.... invertebrate
Invertebrate
An invertebrate is an animal lacking a vertebral column. The group includes 98% of all animal species ? all animals except those in the Chordate subphylum vertebrate .... s, consisting of the three phyla Kinorhyncha
Kinorhyncha
Kinorhyncha is a phylum of small marine body cavity invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos.... , Priapulida
Priapulida
Priapulida are a Phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian.... , and Loricifera
Loricifera
Loricifera is a small phylum of marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more which have been collected and not yet described.... . The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle (ecdysis
Ecdysis
Ecdysis is the molting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed.... ). Their closest relatives are thought to be the Panarthropoda
Panarthropoda
Panarthropoda is a taxon combining the Phylum Arthropoda, Tardigrada and Onychophora. Originally, they were considered to be closely related to the annelids, grouped together as the Articulata, but newer studies place them among a group called the Ecdysozoa.... , Nematoda and Nematomorpha
Nematomorpha
Nematomorpha are a phylum of parasitic animals which are morphology and ecology similar to nematode worms, hence the name. They range in size from 1cm to 1 meter long, and 1 to 3 millimetres in diameter.... ; they are thus placed in the group Ecdysozoa
Ecdysozoa
The Ecdysozoa are a grouping of protostome animals, including the Arthropoda , roundworm, and several smaller phylum . They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes.... .
Markuelia is a genus of fossil worm-like Bilaterias allied to Ecdysozoa and known from strata of Lower Cambrian to Lower Ordovician age. There are two known species, Markuelia hunanensis and Markuelia secunda, that the genus encompasses; the closest known relatives are Loricifera, Kinorhyncha and Priapulida.... , known from fossilized embryos from the middle Cambrian
Cambrian
The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period .... , are thought to be stem Scalidophorans.
The group has also been considered a single phylum, Cephalorhyncha, with three classes.
