INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOANS (EUMETAZOA)

PHYLUM CEPHALORHYNCHA (CEPHALORHYNCHA)

CLASS LORICIFERA (LORICIFERA)

These are marine benthic animals inhabiting the spaces between sand grains, to which they adhere tightly. These animals were first discovered by the Danish researcher R. Kristensen in 1983. They are distributed in seas and oceans across various depths, having been found from 20 to 500 m deep. Research on this group has just begun, yet two families, three genera, and nine species have already been described, the first of which is Nanaloricus mysticus.

Loriciferans are extremely small organisms with a body length of 225–385 µm (and a width of 90 µm). Their body is divided into introvert and trunk regions. The introvert comprises a Mouth cone surrounded by eight to nine pointed stylet-like spines, a middle section armed with nine rows of scalids, and a cervical region with neck scalids. The trunk is enclosed in a cuticular armor, or lorica, consisting of six plates: one dorsal, one ventral, and four lateral. The armor plates feature a fine honeycomb-like sculpture. Glandular ducts open through pores On the surface of the armor. At the posterior end of the lorica are specialized glands that simultaneously function as Sensory Organs, known as flosculi (Fig. 215).

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Fig. 215. Loriciferans — adult female (a) and larva (b) of Nanaloricus mysticus:

1 — mouth cone; 2 — scalid-armed middle part of the introvert; 3 — neck region of the introvert; 4 — trunk armor

The internal anatomy of loriciferans remains insufficiently studied due to their microscopic size, though general features are already known. The mouth opens at the apex of the mouth cone; the Pharynx features a muscular bulb expansion, followed by the Esophagus, a spacious midgut, and the hindgut, with the anus opening at the posterior end of the body. A pair of Salivary Glands opens into the pharynx. The excretory system is represented by a pair of protonephridia.

The Nervous System consists of a circumoesophageal nerve ring and a ventral nerve cord, featuring several ganglia. The circumoesophageal ring incorporates a bilobed dorsal ganglion and eight scalid-innervating ganglia, which are presumably chemosensory organs. The ventral cord bears a large ventral ganglion in the cervical region, several trunk ganglia, and a large anal ganglion located close to the anus.

Loriciferans are gonochoric animals, with females possessing paired Ovaries and an unpaired seminal receptacle, and males having — paired Testes. Details of the Reproductive System's Structure are yet to be clarified; copulatory spines have been identified in males. Their embryonic development is also unstudied, though their larvae are known. The larva differs from the adults by having an unarmed mouth cone, a longitudinally folded lorica, and specialized locomotor appendages. The larvae undergo periodic molting.

R. Kristensen described loriciferans as a novel animal phylum, closely related to cephalorhynchs. However, V. V. Malakhov believes that they combine organizational traits of priapulids and kinorhynchs, uniting with them and hairworms into a single phylum — Cephalorhyncha.