Editing Rotifer (section)

== Anatomy ==
[[File:Figure 28 03 05.jpg|thumb|upright=1.5| {{center|Anatomy of a bdelloid rotifer}}]]

Rotifers have [[bilateral symmetry]] and a variety of different shapes. The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical. There is a well-developed [[cuticle]], which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively called ''loricate'' and ''illoricate''. Rigid cuticles are often composed of multiple plates, and may bear spines, ridges, or other ornamentation. Their cuticle is nonchitinous and is formed from sclerotized proteins.

The two most distinctive features of rotifers (in females of all species) are the presence of ''corona'' on the head, a structure [[cilia]]ted in all genera except ''Cupelopagis'' and presence of mastax. In the more primitive species, the corona forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, however, this has evolved into a more complex structure.

Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, and either expansion or loss of the ciliated band around the head. In genera such as ''[[Collotheca]]'', the corona is modified to form a funnel surrounding the mouth. In many species, such as those in the genus ''[[Testudinella]]'', the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the [[bdelloid]]s, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 272–286|isbn= 978-0-03-056747-6}}</ref>

The trunk forms the major part of the body, and encloses most of the internal organs. The foot projects from the rear of the trunk, and is usually much narrower, giving the appearance of a tail. The cuticle over the foot often forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot partially or wholly into the trunk. The foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, and may even be absent.<ref name=IZ/>

=== Nervous system ===
Rotifers have a small cerebral ganglion, effectively its brain, located just above the mastax, from which a number of nerves extend throughout the body. The number of nerves varies among species, although the nervous system usually has a simple layout. <ref name=IZ/>

The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.<ref name="wallace" />

Rotifers typically possess one or two pairs of short [[antenna (biology)|antennae]] and up to five eyes. The eyes are simple in structure, sometimes with just a single [[photoreceptor cell]]. In addition, the bristles of the corona are sensitive to touch, and there are also a pair of tiny sensory pits lined by cilia in the head region.<ref name=IZ/>

=== Retrocerebral organ ===
Despite over 100 years of research, rotifer anatomy still has many poorly understood components. One of the more mysterious organs in rotifers is the "retrocerebral organ" (RCO), which still remains very enigmatic in its morphology, function, development, and evolution. Lying close to the brain, this organ usually consists of one or more glands and a sac or reservoir. The sac drains into a duct before opening through pores on the uppermost part of the head. Current data shows a wide diversity in structure and potential function. <ref>Fontaneto, D., & De Smet, W. H. (2015). Rotifera, chapter 4 Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifera, Volume 3, Gastrotricha and Gnathifera Schmidt-Rhaesa, Andreas.</ref>  In some species it is reduced or may even be absent completely. [[Benthic zone|Benthic]] species have larger RCO's than [[Plankton|planktonic]] species. Despite this diversity, positional correspondence of RCOs strongly suggests [[Homology (biology)|homology]].<ref name=IZ/><ref name="wallace">
Robert Lee Wallace.
[https://academic.oup.com/icb/article/42/3/660/724027/Rotifers-Exquisite-Metazoans1 "Rotifers: Exquisite Metazoans"].
2002.
{{doi | 10.1093/icb/42.3.660 }}
quote:
"What is the function of the retrocerebral organ?"
</ref><ref name=Hochberg/>

A 2023 study using [[transmission electron microscopy]] and [[Confocal microscopy|confocal laser scanning microscopy]] has illuminated the fine structure of this organ further. The study, the first of its kind, investigated the RCO in one species, ''Trichocerca similis''. It was determined to be a [[Syncytium|syncytial organ]], composed of a [[Anatomical terms of location|posterior]] glandular region, an expansive reservoir, and an anterior [[Duct (anatomy)|duct]]. The [[Gland|glandular]] portion has an active cytoplasm with paired [[Cell nucleus|nuclei]], abundant [[Endoplasmic reticulum|rough ER]], [[Ribosome|ribosomes]], [[Golgi apparatus|Golgi]], and [[Mitochondrion|mitochondria]]. Secretion granules accumulate at the [[Anatomical terms of location|anterior]] end of the gland where they undergo [[Cell fusion|homotypic fusion]] to create larger granules with numerous "mesh-like" contents.  These contents gradually fuse into tubular secretions that accumulate in the reservoir, awaiting secretion. [[Striated muscle tissue|Cross-striated longitudinal muscles]] form a partial sleeve around the reservoir and may function to squeeze the secretions through the gland's duct that often penetrates through the cerebral ganglion.<ref name=Hochberg> Hochberg, R., Araújo, T. Q., Walsh, E. J., Mohl, J. E., & Wallace, R. L. (2023). Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta). Invertebrate Biology, 142(1), e12396. https://doi.org/10.1111/ivb.12396.</ref>
====Retrocerebral organ secretions====
Much like the organ itself, the precise function and biochemical makeup of the secretions is still unknown. The small size of rotifers and small volume of the secretions makes isolation immensely difficult. The secretions have some similarities to the hydrogel secretions that form gelatinous housings in some rotifer species.
[[Ultrastructure]] analysis of ''T. similis'' secretions showed them to be a series of tube-like secretions with a highly filamentous framework. This is highly suggestive of a [[glycosaminoglycan]] structure- proteins with negatively charged [[polysaccharide]] chains forming [[proteoglycan]] molecules. These molecules are standard in vertebrate and invertebrate [[Gelatin|gelatins]] such as [[mucus]]. <ref name=Hochberg/> 

Despite recent advancements in understanding RCO organ and secretion ultrastructure, the exact function of the organ is still ultimately unclear. The leading hypotheses are that the RCO secretes a mucus-like substance that aids in [[Benthic boundary layer|benthic]] locomotion, adhesion, and/or reproduction (i.e., attachment of eggs to a substrate), although more research is needed to explore function and evaluate the homology between species.<ref name=Hochberg/>

=== Digestive system ===
[[File:Bdelloid.JPG|thumb|upright=1.5|right|Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.]]

{{Multiple image
| direction = vertical
| align = right
| width = 220
| header = Rotifer colonies
| image1 = 20100708 colonial rotifer Conochilius.JPG
| caption1 = Colonial rotifers, tentatively identified as ''Conochilus''. The colony is less than 1 mm in diameter, but visible to the naked eye.
| image2 = Sinantheria-socialis.jpg
| caption2 = Colony of ''Sinantheria socialis'' on an ''[[Elodea densa]]'' leaf. Note heart-shaped corona of individuals.
}}

The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing [[pharynx]] (called the ''mastax''), sometimes via a ciliated tube, and sometimes directly. The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called ''trophi'', which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending partly on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like [[forceps]] to help bite into prey. In some [[ectoparasite|ectoparasitic]] rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.<ref name=IZ/>

Behind the mastax lies an [[oesophagus]], which opens into a [[stomach]] where most of the digestion and absorption occurs. The stomach opens into a short [[intestine]] that terminates in a [[cloaca]] on the posterior dorsal surface of the animal. Up to seven [[salivary gland]]s are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that produce [[digestive enzyme]]s.<ref name=IZ/>

A pair of [[protonephridium|protonephridia]] open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain [[osmoregulation|osmotic balance]].<ref name=IZ/>

{{Multiple image
| caption_align = center
| direction = horizontal
| align = left
| header = Individual rotifers
| total_width = 660
| image1 = Philodina rugosa 756859.jpg
| caption1 = ''Philodina rugosa''
| image2 = Mikrofoto.de-Raedertier Ptygura pilula 2.jpg
| caption2 = ''Ptygura pilula''
| image3 = Mikrofoto.de-Brachionus quadridentatus 6.jpg
| caption3 = ''[[Brachionus|Brachionus quadridentatus]]''
}}
{{clear}}
===Biology===
The coronal [[cilia]] pull the animal, when unattached, through the water.

Like many other microscopic animals, adult rotifers frequently exhibit [[eutely]]—they have a fixed number of [[cell (biology)|cell]]s within a species, usually on the order of 1,000.

Bdelloid rotifer genomes contain two or more divergent copies of each [[gene]], suggesting a long-term asexual [[evolution]]ary history.<ref>{{cite journal |author= Jessica L. Mark Welch, David B. Mark Welch & Matthew Meselson |date=10 February 2004 |title=Cytogenetic evidence for asexual evolution of bdelloid rotifers |journal=[[Proceedings of the National Academy of Sciences]] |volume=101 |issue=6 |pages=1618–1621 |doi=10.1073/pnas.0307677100 |pmid=14747655 |pmc=341792 |bibcode=2004PNAS..101.1618W |doi-access=free }}</ref> For example, four copies of hsp82 are found. Each is different and found on a different chromosome excluding the possibility of [[homozygous]] [[sexual reproduction]].