INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION ECDYSOZOA

PHYLUM CEPHALORHYNCHA

CLASS LORICIFERA

Marine benthic animals inhabiting various depths (from 20 to 500 m) in the interstitial spaces of the substrate, adhering tightly to sand grains. The first loriciferan species (Nanaloricus mysticus) was described by the Swedish zoologist A. Kristensen in 1983. Currently, 14 species of these animals are known.

Anatomy. Minute (0.2-0.4 mm) organisms. Their body is divided into an introvert (proboscis) and a trunk region. The introvert comprises the oral cone surrounded by pointed spine-like stylets, a middle region armed with nine rows of hooks (scalids), and a neck region with neck scalids. Scalids can perform sensory, locomotory, and prey-capturing Functions. In adults, the introvert can be retracted into the trunk.

The trunk is enclosed in a cuticular armor known as a lorica (from the Latin lorica, meaning breastplate or armor, which gives the phylum its name). The lorica consists of six plates: dorsal, ventral, and four lateral (Fig. 284). The surface of the plates features a fine honeycomb-like sculpture and anteriorly directed hollow spines. Glandular ducts open onto the surface of the armor. Glandular structures called flosculi are located on the posterior part of the lorica and function simultaneously as Sensory Organs.

Class="center">

Fig. 284. Loriciferans (after Malakhov): a - adult female; b - larva of Nanaloricus mysticus:

1 - oral cone; 2 - middle region of the introvert armed with scalids; 3 - neck region of the introvert; 4 - trunk armor

The body cavity is a schizocoel.

Digestive System. The Mouth opens at the tip of the oral cone; the Pharynx features a muscular bulb, followed by the Esophagus and a voluminous midgut. The hindgut terminates in an anus 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 circumoral nerve ring and a ventral nerve cord. A bilobed dorsal ganglion and eight ganglia innervating the scalids are located on the circumoral ring. Along the ventral cord, There is a large ganglion in the neck region, followed by several smaller trunk ganglia, and a large anal ganglion near the anus.

Sensory organs, besides the flosculi, presumably include the scalids, which are considered to function as chemoreceptors.

Reproductive System. Loriciferans are dioecious animals. Females possess paired Ovaries and a single seminal receptacle, whereas males have paired Testes and copulatory spines.

Reproduction and development. Development proceeds with metamorphosis. Unlike the adults, the larval oral cone is unarmed, its armor exhibits longitudinal folding, and growth is accompanied by periodic molting. The larva features characteristic appendages: two to three pairs of spines on the anteroventral part of the armor and a pair of movable tail appendages.