Etymology: From the Greek Di for two and cyem for germ – referring to the existence of both sexual and asexual reproduction in this group.
Characteristics of Dicyemida:
- Bilaterally symmetrical.
- Has no organs or tissues.
- Body contains no internal cavity.
- Body possesses no digestive tract (gut).
- Body only two cell layers in most places.
- Has no nervous system.
- Has some cells develop inside other cells.
- Reproduction quite complex involving both sexual and asexual aspects.
- All are endoparasites on other marine invertebrates (Mollusca; Cephalopoda).
According to the online database the Catalogue of Life there are 122 living species of Dicyemida
The Dicyemida were once part of the Mesozoa. However modern phylogenomic analyses suggest the Dicyemida and the Orthonectida are two separate phyla. See the discussion on Mesozoa, or Zverkov et. al. May 2019.
The Dicyemida are a small phylum of small (0.5 to 7 millimetres in length) and poorly understood parasitic marine animals. They have very simple bodies, often consisting of less than 50 cells. All known species are internal parasites cephalopods. They are sometimes referred to as the Rhombozoa. The Dicyemida have no gaseous exchange organs, no circulatory system, no nervous system and no digestive system.
The Dicyemida are all parasites of cephalopods (Octopus and Squid), they live in the animal’s renal system, their kidneys.
Dicyemida have a more complicated life cycle than their cousins the Orthonectida. Their basic body plan is a long, thin central cell called an axial or tube cell, surrounded by a coat of smaller ciliated cells which are arranged spirally around the axial cell. Some authors equate this with a two cell-layer body plan.
The axial cell contains smaller cells called axoblasts. These axoblasts give rise to either, vermiform (long and thin) asexual larvae called nematogens sexually reproducing individuals called rhombogens.
The two forms are physically identical as far as we know. The only difference being that in the nematogen stage, the axoblasts produce more nematogens and in the rhombogen stage they produce infusorigens which serve as the animal’s gonads (organs which produce eggs and sperm).
The eggs are fertilised inside the axial cell where they develop into infusoriform larvae which quickly develop the adult number of cells. Each species has a definite number of cells in its adult form. This infusoriform larvae then leaves the axial cell and the hosts body, with its urine. They then sink to the sea floor where they grow by means of cell enlargement, rather than by cell addition. It is not currently know how these larvae re-enter their hosts and become nematogens.
The phylum Dicyemida contains only a single class, Rhombozoa and no orders.
Phylum Dicyemida; Class Rhombozoa
- Family Conocyemidae • 2 living spp
- Genus Conocyema • 1 living spp
- Genus Microcyema • 1 living spp
- Family Dicyemidae • 119 living spp
- Genus Dicyema • 69 living spp
- Genus Dicyemennea • 42 living spp
- Genus Dicyemodeca • 3 living spp
- Genus Dodecadicyema • 1 living spp
- Genus Pleodicyema • 1 living spp
- Genus Pseudicyema • 3 living spp
- Family Kantharellidae • 1 living spp
- Genus Kantharella • 1 living spp
Image Credits:- Cover image (Dicyema japonicum) by Hidedaka Furuya – License CC BY-SA 4.0
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