Kinorhyncha

The chinorhynchos (Kinorhyncha, from the Greek kinema, "movement" and rhynchos, "trunk") are a phylum of small marine invertebrates (0.13 to 1.04 mm) that inhabit benthic environments around the world.

To date, more than 250 species have been described, inhabiting mainly muddy or muddy-sandy substrates, where they move thanks to the action of special spines (schalides) present on the front part of the body.

In ancient times, chinorhynchuses were grouped in the polyphyletic group Aschelminthes due to the presence of pseudocoelom. Today, however, they are part of the Ecdysozoa clade. Together with the priapulids and loricifers, they form the clade Scalidophora (=Cephalorhyncha).

General morphology

The body of a chinorhynchus is divided into head, neck and a trunk made up of eleven segments. The segmentation of the body is reflected in the cuticular plates, spines, the nervous system and its associated structures (mainly sensory patches and papillae), and in the musculature. The head and neck do not show a segmental organization, which is why they are not considered segments sensu stricto.

Head

The head is formed by an eversible introvert and a protrusible oral cone, which can retract into the trunk. Through it, these animals fulfill the functions of feeding and locomotion. In turn, the appendages present on the head also act as sensory structures.

The introvert is formed by the schalides, which are locomotor and sensory appendages, external to the oral cone, arranged in up to seven rings. Ring 01 (the innermost one) carries 10 primary spinoschalids, characterized by being much longer and thicker than the rest of the skalides. Rings 02 to 06 carry five, ten or 15 spinoscalides, shorter than those of ring 01. The last ring (07) is formed by six, nine or 14 tricoscalides; They are the shortest scalides and their surface is covered by very fine hairs.

Inside the introvert is the oral cone, protrusible, which gives entrance to the digestive tube. It is made up of smaller appendages called oral stylets, arranged in four rings. The outermost one (ring 00) is made up of nine external oral stylets, while the innermost ones (-01 to -03) are made up of 20 internal oral stylets (5 + 5 + 10, inside out). The deeper internal oral stylets (ring -03) are called helioscalides, because their base has a radiated appearance (cuticular projections). The external oral stylets show great variability between species in terms of shape, size, number of gears and rigidity.

The oral stylets and schalides (with the exception of the tricoscalides) are arranged in a pentarradial pattern, dividing the introvert into ten defined sectors (s1 to s10).

Neck

The neck is formed by a series of small cuticular plates, the placides, which constitute a closing apparatus when the introvert retracts in the trunk. Their shape and number vary between species. In most of them, they articulate at their base with the anterior edge of the first segment of the trunk, but in some taxa they can be fused with that segment or even missing (Cateria, Franciscideres). The number, shape and development of the placids varies according to the genus, between 16 and seven, arranged radially or bilaterally.

Trunk

The trunk is made up of eleven segments (also called zonitos). Species of the class Cyclorhagida have an elongated, fusiform trunk with a circular, heart-shaped or oval section; while the species of the class Allomalorhagida are larger, with a rectangular body and triangular section, except for the families Dracoderidae and Franciscideridae, whose general morphology fits that of cyclorrhagids.

The cuticle of each segment is divided into dorsal (tergal) and ventral (sternal) plates, which vary between one and four per segment. The number and arrangement of the plates and other cuticular structures have great taxonomic importance at the family and genus levels. The tergal and sternal plates articulate laterally forming the tergosternal junction, while the sternal plates articulate with each other in the mediosternal junction. If the segment consists of a single tergal plate that curves ventrally, the two edges join at the medioventral joint. In most families there are thickenings on the anterior edge of each plate called pachycycli, which act as a site of muscle fixation.

In addition, each segment is equipped with various external cuticular structures, the shape, number and distribution pattern of which are of great taxonomic importance. The most characteristic structures are the spines, which extend from segments 1 to 10 in a dorsal position. and/or lateral. In the last segment there are usually long, robust acicular spines: two lateroterminal, two accessory lateroterminal, or one medioterminal. In turn, in some segments there may also be tubes (adhesive or sensory), setae and hairs.

Of great importance is the presence of pores (with different functions) and sensory spots (sensory spots). The latter are cuticular specializations that allow the individual to receive sensory stimuli. They appear as a rounded, oval or drop-shaped area, with one, two and in some cases three pores, and with numerous micropapillae (up to 100). There are 5 types of sensory stains.

• Type 1. They have two pores, one central and the other side, which open at the same height.
• Type 2. They also have a central pore and a lateral pore, the central one is surrounded by a micropapel ring, while the side is on a short cuticular tube.
• Type 3. They are formed by a conical base that at its end carries a few micropapels. This type of sensory stain also receives the name of flosculumas it recalls the sensory organs of Priapulida and Loricifera.
• Type 4. They are formed by a circular patch of numerous micropapals with an elongated tube carrying a pore.
• Type 5. They are formed by a single ring of micropapilas (casas) surrounding a central pore. They also receive the name of Nonaloricus-flosculum or N-flosculum, because a similar receptor is in the genus of loricfers with the same name.

Taxonomy

The phylum Kinorhyncha is represented by more than 250 species, distributed in two classes, eleven families and 30 genera. The following classification is based on Sorensen et al.and all derivative works. The group is divided into two classes: Cyclorhagidawith orders Echinorhagata (1 family), Kentrorhagata (5 families) and Xenosomata (1 family); and Allomalorhagida (4 families).

Cyclorhagida class

• Order Echinorhagata
  • Echinoderidae
    • Gender Cephalorhyncha
    • Gender Echinoderes
    • Gender Fissuroderes
    • Gender Meristoderes
    • Gender Polacanthoderes
• Order Kentrorhagata
  • Family Zelinkaderidae
    • Gender Triodontoder
    • Gender Zelinkaderes
  • Family Centroderidae
    • Gender Centreders
    • Gender Condyloderes
  • Semnoderidae
    • Gender Semnoders
    • Gender Sphenoderes
  • Antygomonidae
    • Gender Antygomonas
  • Family Cateriidae
    • Gender Cateria
  • Incertae sedis
    • Gender Tubulideres
    • Gender Wollunquaderes
• Order Xenosomata
  • Family Campyloderidae
    • Gender Campyloderes
    • Gender Ryuguderes

Class Allomalorhagida

• Family Dracoderidae
  • Gender Dracoderes
• Family Franciscideridae
  • Gender Franciscideres
  • Gender
• Pycnophyidae
  • Gender Pycnophyes
  • Gender Leiocanthus
  • Gender Higginsium
  • Gender Cristaphyes
  • Gender Fujuriphyes
  • Gender Krakenella
  • Gender Setaphyes
• Family Neocentrophyidae
  • Gender Mixphys
  • Gender Neocentrophys
  • Gender Paracentrophys