Gastrotrich model
Gastrotricha

External Features

Gastrotrichs are free-living microscopic worms. They range in length from 0.1 to 1.5 mm, but are usually less than 0.6 mm long.
They inhabit fresh-water and salt-water. Gastrotrichs are elongated, ventrally flattened and usually bristly or spiny. They glide
on ventral cilia.

The head may be rounded and lobe-like and is joined to the trunk via a constricted neck. The posterior end is forked, pointed,
rounded, truncate or drawn out into a slender tail. There are 1-2 pairs of
red pigment spots (ocelli?) on the sides of the head
lobe in some Macrodasyoidea. Some have 1-2 pairs of
tentacles or palps.

The dorsal surface is convex or flattened. Some gastrotrichs are nearly circular, with convex dorsal and ventral surfaces.
Gastrotrichs are colourless and transparent; although ingested food may give colour to the gut.

Cilia occur on the head lobe and usually also on the entire ventral trunk surface. Macrodasys has one ventral broad ciliary
band running the entire length of the animal.
Chaetonotus and Turbanella have 2 narrow ventral longitudinal bands of cilia. In
Dasydytes and Setopus small ciliated patches form two longitudinal ventral rows. Thaumastoderma has transverse ventral
bands of cilia. Cilia may be confined to the anterior part of the trunk or they may be absent. The ventral trunk cilia usually
continue onto the ventral surface of the head lobe, which is ciliated on both surfaces with sparse cilia or tufts, patches,
transverse groups or partial girdles of cilia.

There may be very motile or immotile long cilia or bristles on the head, especially in the chaetonotoids, which may have a
sensory function. In freshwater genera there are often 2-3 tufts or short bands of these long cilia / bristles on each side of the
head lobe, as in
Chaetonotus, Dasydytes, Ichthydium and Stylochaeta, and others.

In some gastrotrichs there are 1-2 rows of long cilia on the dorsal surface of the head lobe, as, for example, in
Turbanella and
Proichthydium. In Xenotrichula the cilia of the ventral surface of the head and trunk bundle together to form cirri that are
employed in running about in a similar manner to hypotrichous
ciliates.

Body Surface

The body surface is covered in a thin cuticle, which usually forms surface structures such as warts or scales (flat or
overlapping) that may be smooth or each equipped with a median keel or posteriorly curved spine. Spined scales are
characteristic of the chaetonotoids. The
spines may subdivide into 4-5 prongs, as in Thaumastoderma and Tetranchyroderma.
Xenotrichula and Aspidiiophorous have double-decked stalked spines, each consisting of 2 superimposed scales connected by
a stalk.
Dasydytes has spines, but no scales.

Stylochaeta and Dasydytes have very long spines in bunches on the body sides. Neogossa has such spines on its posterior
end. Marginal scales may have longer spines. Marginal spines are sometimes the only spines present.

Lepidodermella ( = Lepidoderma) has a complete covering of spineless scales. Plates may be formed from the fusion of scales,
especially on the ventral surface and the head lobe of chaetonotoids. The head plates that are usually present in this instance
are the unpaired cephalion anterior to the mouth, unpaired hypostomium behind the mouth and a pair of lateral pleurions. Each
of the lateral pleurions may be subdivided.

Adhesive Tubes. Adhesive tubes are most abundant in the Macrodasyoidea, and are also present in the Chaetonotoidea,
except in the pelagic families Dasydytidae and Neogosseidae, These adhesive tubes are cylindrical projecting cuticular tubes
and each is movable by one or more muscles and is supplied by a gland cell with 1-3 nuclei. These tubes function to adhere the
animal to objects.

There are up to 250 adhesive tubes in macrodasyoids, arranged in longitudinal series at or near to the lateral margins, and in
longitudinal or transverse rows or clusters on the ventral surface of the head and in bunches on the tail forks or along the sides
of unforked tails.

In
Turbanella, each lateral tube is accompanied by a long motile sensory cilium. Chaetonotoids have 1-2 on each tail fork only.
These may comprise the main part of the tail fork or are terminal on the forks as toes.

Epidermis. The epidermis is syncytial and mostly thin, but is much thickened laterally in macrodasyoids. Epidermal glands form
the adhesive glands of the adhesive tubes and the adhesive dorsal glands of the macrodasyoids. These dorsal glands are
numerous dorsolateral and rounded bodies of granular or homogeneous material, each with a pore.

There is no subepidermal muscle sheath, but delicate anucleate circular fibres, which are continuous with the epidermis and
situated in or just beneath the epidermis, give rise to the muscles of the lateral adhesive tubes and moveable bristles.

Longitudinal muscles comprise the ventrolateral group that extends the length of the body and operates the anterior and
posterior adhesive tubes and inserts on the mouth region and pharynx. There are also 2-6 smaller dorsal longitudinal bands,
situated along most or part of the dorsal wall and inserting on the mouth region and the pharynx. The longitudinal muscle fibres
are nucleated and smooth, except in
Dactylopodalia, in which they are striated.
Pseudocoelom

The pseudocoel consists of small spaces between the body wall and the viscera and has no definite lining. In the
macrodasyoids membranes derived from the epidermis divide the pseudocoel. These membranes may contain muscle fibres.
These divisions give rise to a central compartment around the digestive tract and ripe eggs, and paired lateral spaces
containing the gonads. There are no free amoeboid cells in the pseudocoel.

Nutrition

The mouth is terminal or slightly ventral and may be bordered by numerous small curved hooks. In chaetonotoids the mouth
opens into a short cuticular buccal capsule bearing longitudinal ridges and sometimes bearing projecting teeth. This buccal
capsule is protrusible to some extent and leads into the pharynx. In the macrodasyoids, the thin-walled distal end of the pharynx
possibly comprises a buccal capsule.

The pharynx is very similar to the nematode pharynx. It is an elongated tube one sixth to one third of the body length. It is
usually equipped with 1-4 bulbous enlargements (4 in
Neogossea) and has a 3-angled lumen. In chaetonotoids there is one
midventral angle and 2 dorsolateral angles, as in nematodes, but in macrodasyoids there is one middorsal and 2 ventrolateral
angles.

The wall of the pharynx consists of columnar epithelium containing cross-striated radial muscle fibres that are best developed in
the bulbous regions. These fibres are part of the epithelium. Some of the epithelial cells are modified into gland cells. In some
chaetonotoids, the pharynx has 2 pairs of large projecting cells, which possibly function as salivary glands.

The pharynx lumen has a thin cuticular lining. The pharynx has an external covering of thin circular muscle fibres, which are
epithelial extensions that form an oral sphincter at the mouth.

In macrodasyoids a pair of pharyngeal pores connect the pharyngeal lumen to the outside. The posterior of the pharynx often
projects into the midgut as a pharyngeal plug. The pharynx leads into the midgut.

The gastrotrich midgut, or stomach-intestine, is a simple straight tube with no external glands. It consists of a wider anterior
stomach and a narrower posterior intestine, although these regions are not definitely delimited. A single layer of cuboidal or
columnar epithelium lines the lumen. The midgut has no cuticle lining and has a thin external covering of circular muscle fibres.
Gland cells are apparently present, especially in the anterior stomach, and these possibly secrete digestive enzymes.

The anus is ventral in macrodasyoids and situated between the bases of the tail forks, but is terminal or slightly dorsal or
slightly ventral in chaetonotoids. The anus is lined by cuticle and is sometimes equipped by an anal sphincter muscle.

Digestion presumably occurs in the midgut lumen. The food comprises bacteria, protozoans, diatoms and detritus, etc. Food is
ingested by the sucking action of the pharynx whilst the animal continuously moves about. Some chaetonotoids attach to the
substrate by the adhesive tubes of their tail forks and gather food particles by ciliary action.

Excretion

Chaetonotoids have one pair of protonephridia, with one protonephridium either side of the middle of the digestive tract. Each
consists of a single non-nucleated flame bulb with a very long flame. From each bulb a highly coiled tubule connects to the
outside, via the nephridiopore on the ventral side of the middle of the body. There is no urinary bladder.

The macrodasyoids have no protonephridia, but they have one or more pairs of granular masses that open to the outside
ventrally (ventral glands) and are possibly excretory.
Nervous System

Gastrotrichs have a relatively large brain consisting of a mass either side of the anterior pharynx with a broad or narrow dorsal
connection between them. The brain gives out a pair of lateral nerves that contain ganglion cells and extend the length of the
body.

Sensory Systems

The tufts of long cilia on the chaetonotoid head lobe are continuous with ganglion cells in the brain and are presumably
sensory. Single stiff or motile tactile hairs or bristles are scattered over the body and are more numerous in the macrodasyoids.

The head possesses lateral sense organs. In chaetonotoids these are a pair of ciliated pits on the head lobe just behind the
most posterior ciliary tufts. In many macrodasyoids the lateral cephalic sense organs are a pair of piston pits, in which the
bottom of the pit contains an unciliated projection, the piston. Disappearance of the pit and cilia and elongation of the piston
has given rise to the lateral tentacles or palps in some genera, e.g.
Thaumastoderma, Neogossea and Xenotrichula.

Pigment ocelli are found in a few species. These are aggregations of red pigment granules inside some of the brain cells.
Groups of highly refringent bodies found in some brain cells or in the cells of the ciliated pits possibly have a static function.

Locomotion

Locomotion is by ciliary gliding, by use of the ventral cilia. This is extraordinarily graceful, and quite fast, and a joy to watch
under the microscope. Macrodasyoids can locomote by ciliary gliding and leech-like movements. The anterior and posterior
adhesive tubes are employed in the leech-like mode of locomotion. They also exhibit long periods of temporary sessility whilst
attached to the substrate by their posterior adhesive tubes.

Reproduction

Gastrotrichs are thought to have been originally hermaphroditic, but the male system has degenerated in Chaetonotoidea,
which consist of females only (except for
Xenotrichula) that reproduce by parthenogenesis. The Macrodasyoidea are
hermaphroditic, some are protandric and
Dactylopodalia has male, female and hermaphroditic individuals.

There are 1-2 ovaries comprised of cell masses, with no definite capsule, in the posterior part of the body. These contain up to
25 ovocytes, the number is possibly fixed during development of the embryo (?) as no mitoses have been observed.  The eggs
ripen and become free in the uterus anterior to the ovary. In those forms in which the pseudocoel is subdivided into central and
lateral compartments, the larger eggs lie alongside the midgut in the central compartment.

The macrodasyoids have a single oviduct with an enlarged anterior serving as a seminal receptacle and a posterior thicker
walled swelling that forms a copulatory bursa that opens to the outside via the female gonopore. The female gonopore opens
anterior to the anus or in common with the anus.
Turbanella has no copulatory bursa, however, and Macrodasys has no
seminal receptacle.

There is usually a yolk gland present as a single or paired mass of nutritive tissue. Chaetonotoids have no definite oviduct, but
have a posterior organ X that opens to the outside via a ventral pore and is possibly a copulatory bursa. There are traces of
testes in some chaetonotoids.

In macrodasyoids the male system consists of a pair of testes or a single right testis. Each testis is connected to the median
male gonopore via its own sperm duct. There may be a single male gonopore, but there is sometimes a pair of such pores
when two testes are present. The male gonopore(s) opens close to the female gonopore, or in common with it, or the male
gonopore is situated anterior to the female gonopore.
Macrodasys and Urodasys have a [penis-like structure.

Development

Lepidodermella squamatum is a fresh-water chaetonotoid. It lives for 8-21 days and lays up to 5 eggs (usually 3-4). Apparently
all chaetonotoids produce two types of egg (as in rotifers): subitaneous and dormant. Both types of egg develop
parthenogenetically. The eggs are oval and enclosed in a shell, which is thicker in dormant eggs. The shell possesses bristles
or blunt spines, sometimes only on one side. The eggs are laid on the surface film or near an object. They hatch in 1-3 days
and the hatchlings resemble adults and reach sexual maturity in about 3 days. The number of adhesive tubes may increase
after hatching.

In
Neogossa, cleavage is holoblastic and determinate and gives rise to a coeloblastula. During gastrulation two ventral cells
enter the interior and possibly give rise to the midgut. Cells are proliferated into the interior to form the pharynx and the
hindgut. Two ventroposterior cells give rise to the primordial germ cells that develop into the ovaries.

Classification and Ecology

There are about 450 species of described gastrotrich.

O. Macrodasyoidea. These gastrotrichs have elongated vermiform bodies with or without a head lobe. They possess anterior,
lateral and posterior adhesive tubes, pharyngeal pores and a male reproductive system. They lack protonephridia. All
macrodasyoids are marine and live on shores amongst sand or in the vegetation zone. Macrodasyoids have been found along
European coasts, but possibly occur in other parts of the world. Examples of macrodasyoid genera are:
Cephalodasys,
Lepidodasys, Acanthodasys, Macrodasys, Urodasys, Dactylopodalia (Dactylopodella), Turbanella, Thaumastoderma,
Tetranchyroderma
and Platydasys.

O. Chaetonotoidea. Chaetonotids are usually fusiform and possess a head lobe with tufts of long sensory cilia or bristles. They
lack pharyngeal pores. Chaetonotoids have no male reproductive system and are parthenogenetic.
Neodasys and
Xenotrichula are exceptions, as they possess a male reproductive system. There are 1-2 adhesive tubes on the tail forks only,
except in
Neodasys in which there are lateral adhesive tubes, each of which is accompanied by a long cilium. Chaetonotoids
possess one pair of protonephridia. Most are fresh-water and benthic, dwelling among vegetation in ponds, lakes, fouling
material, moss pools and bogs.
Chaetonotus is very common in ponds. Neodasys is marine and resembles macrodasyoids and
has an elongated band-like body, lateral adhesive tubes, adhesive tubes on tail forks and a male reproductive system.
Xenotrichula is also marine, but has the typical chaetonotoid appearance and possesses a head lobe, dorsal stalked scales,
adhesive tubes on the tail forks only and some male reproductive system is retained. Examples of chaetonotoid genera are:
Chaetonotus, Icthydium, Lepidodermella (Lepidoderma), Polymerurus, Heterolepidoderma, Stylochaeta, Aspidophorous,
Proichthydium, Neogossea
and Dasydytes.
Gastrotrichs
A 3D computer model of a gastrotrich of the Chaetonotus type (a fresh
water gastrotrich).