| Rhombozoa/Dicyemida | |
|---|---|
 | |
| Scientific classification | |
| Kingdom: | |
| Superphylum: | |
| Phylum: | Rhombozoa van Beneden, 1876 |
| Families | |
TheRhombozoa, orDicyemida, are a group of tinyparasites that live in therenal appendages ofcephalopods.
Classification is controversial.[1] They are probably a newphylum. Traditionally, dicyemids have been grouped with theOrthonectida in theMesozoa. However,molecular phylogeny indicates that dicyemids may be more closely related to theroundworms.[2]
Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.276 in), and they can be seen through a light microscope.[3] They haveeutely: each adult individual of a givenspecies has the same number of cells. Cell number is a useful identifying character.
The organism's structure is simple: a single axial cell is surrounded by a jacket of twenty to thirtyciliated cells.[3]
Dicyemids exist in bothasexual andsexual forms. The asexual forms are found in juvenile and immature hosts; and the sexual forms in mature hosts.
The asexual stage is termed anematogen; it producesvermiform (worm-like)larvae in the axial cell. These mature through direct development to form more nematogens.[3] Nematogens proliferate in young cephalopods, filling thekidneys. Later, as the hosts mature, sexual forms calledrhombogens are formed. They arehermaphrodites, withgonads of both sexes. The rhombogens self-fertilise, and later releaselarvae.
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