INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - G.Yu. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM CEPHALORHYNCHA

This is a small group of worm-like organisms, the overwhelming majority of which are marine, while representatives of only one Class — Horsehair Worms (Gordiacea) — spend part of their life cycle in marine and freshwater habitats, and another part inside the bodies of Arthropods. Their body size ranges from 225 µm to 1.5 m. Cephalorhynchans share the same level of Organization as rotifers, acanthocephalans, gastrotrichs, and nematodes, which is why they were long classified under the phylum Nemathelminthes, even though this grouping was recognized as artificial.

The body of cephalorhynchans consists of two regions — the HEAD (proboscis) and the trunk. The proboscis comprises the oral cone, a middle section bearing backward-curving hooks (scalids), and a neck region. Ciliated epithelium is absent; the body is covered by a cuticle and possesses a Skin-Muscle sac, beneath which lies a primary body cavity (schizocoel). Cephalorhynchans have a complete, through gut. The excretory system is of the protonephridial type, though it is entirely absent in some Representatives of the phylum. The Nervous system consists of a circumoral nerve ring and an unpaired ventral nerve cord. Cephalorhynchans are dioecious animals; Cleavage of the egg is radial and nearly equal. The anus forms at the site of the blastopore, whereas the Mouth opens independently of the blastopore at the anterior end of the embryo. This characteristic is known as deuterostomy. Cephalorhyncha develop through metamorphosis, hatching from the egg as a larva that differs from the adult. The phylum Cephalorhyncha includes four classes: Priapulida, Kinorhyncha, Gordiacea (or Nematomorpha), and Loricifera. Based on their body plan and embryonic development features, cephalorhynchans stand apart from other pseudocoelomate worms, which led the Russian scientist V. V. Malakhov to elevate them to a separate phylum, which also includes the recently described group Loricifera.