A. Echinoderes discovered in 1841 (Dujardin), considered to be allied to arthropods or annelids, later to nematodes.

B. Separated as Class Kinorhyncha (Reinhard 1887).

C. Monograph by Zelinka (1920).

D. Currently considered as a phylum or as a class in Aschelminthes.

A. Aschelminth characters: pseudocoel, vermiform, unsegmented, cuticle, straight digestive tube, posterior anus.

B. Kinorhynch characters: marine, unciliated, superficially segmented, more or less spinous, wormlike, with a retractile head, and 1 pair of protonephridia.

A. Size less than 1 nun.

B. Bottle-shaped and flattened ventrally. Head, neck, trunk.

C. Color yellowish or brownish.

D. No external ciliation. Cuticle divided into articulated segments or zonites. The spherical head forms the first zonite, the neck another zonite, the trunk 11 (12) zonites. Each segment has a dorsal and 2-3 ventral plates. The former bears median and lateral spines.(scalids).

E. Spines are recurved, movable, hollow, and filled with epidermis. They consist of a basal socket and a blade. May have bristles (trichoscalids).

F. One or more pair(s) of adhesive tubes, with large gland cells at the base, are on the ventral surface of third and fourth zonite.

A. Body, wall: columnar, syncitial epithelium extending into spines, and forming thickenings or chords longitudinally.

B. Muscles: radial, but outside the epithelium. Segmented longitudinal muscles around the pharynx and along the body.

C. Body cavity: well-developed pseudocoel, containing amoeboid cells that originate from the wall of the digestive tract.

D. Digestive tract:

1. Mouth anterior with protrusible cone, surrounded by scalids

                                            and armed with oral styles.

                                        2. Pharynx: lined with cuticle and surrounded by muscle.

                                        3. Esophagus with two pairs of salivary glands, and two
                                            pancreatic glands.

                                        4. Stomach-intestine: nonciliated columnar epithelium.

                                        5. Hindgut and terminal anus. Sphincter before hindgut and anus.

F. No special respiratory structure. Two protonephridia with single, modified flame bulb and short tubes. Nephropores on dorsolateral surface of 11th segment.
 

G. Nervous system closely associated with epidermis:

1. Brain a nerve ring_,

2. A midventral nerve cord, with one ganglion per segment. Other ganglia are not connected to form a cord.

H. Sense organs: sometimes ocelli, sensory bristles on body surface
 
I. Reproductive system:

1. Dioecious, but little sexual dimorphism (size, minor external details).

2. Females have a saclike ovary in the middle of the body, a short oviduct, a gonopore on the last segment.

3.  Males have 1 pair of testes and one pair of gonopores similarly located, plus 2-3 penial spicules.

A. Internal fertilization, at all seasons of year.

B. 3-6 eggs, developing externally.

C. Young not segmented, without spines and head, and with an incomplete digestive tract. They undergo successive molts before resembling adults.

A. Marine, benthic.

B. Burrow in mud of coastal waters but some were found at 5000m depth.

C. Feed on diatoms or deposit-feed. 

D. Worm-like movements with the aid of scalids or by contractions of the head.

E. Zooxanthellae in epidermis or other tissues.

Cyclorhagae: only the first zonite is retractable. The second zonite forms the closing apparatus. Examples: Echinoderes, Echinoderella, Centroderes, Campyloderes.

Conchorhagae: the third zonite forms the closing apparatus, i.e. a pair of shells. One genus: Semnoderes.

Homalorhagae: third zonite a closing apparatus with one dorsal and three ventral plates. Few spines, more truncate shape. Examples: Pycnophyes, Trachedemus.

VIII. Phylogeny

A. Unique characteristic: segmentation extending into the musculature and the nervous system. For this they have been related to arthropods and annelids. However in Kinorhyncha the segmentation progresses inward from the cuticle, and the organization is simpler.

B. They bear similarities to sipunculids, acanthocephala, rotifers, tardigrades, nematodes. Gastrotrichs, kinorhynchs, and nematodes are considered as offshoots from a comnon stem.

IX. References

Barnes R., 1974 : Invertebrate zoology, 3rd ed. Saunders, Philadelphia. pp 208-210.

Gosner K.L., 1971 : Guide to the identification of marine and estuarine invertebrates. Cape Hatteras to the Bay of Fundy. Wiley and Sons, NY. pp 195-197.

Hyman L., 1951 : The invertebrates. Vol. III. McGraw-Hill, NY. pp 53-59, pp 170-183.

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