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(Leuckart, 1894)

Outline classification
Phylum Chaetognatha (Leuckart, 1894)
Class Sagittoidea Claus and Grobben, 1905
Subclass Syngonata Casanova, 1985
Order Biphragmophora Casanova, 1985
Family Heterokrohniidae Casanova, 1985
Genus Heterokrohnia Ritter-Záhony, 1911
Genus Archeterokrohnia Casanova, 1986
Genus Xenokrohnia Casanova, 1993
Subclass Chorismogonata Casanova, 1985
Order Monophragmophora Casanova, 1985
Family Spadellidae Tokioka, 1965
Genus Spadella Langerhans, 1880
Genus Paraspadella Salvini-Plawen, 1986 (sensu Bowman and Bieri, 1990)
Genus Hemispadella Casanova, 1996
Family Eukrohniidae Tokioka, 1965 (emend.)
Genus Eukrohnia Ritter-Záhony, 1909
Genus Bathyspadella Tokioka, 1939
Order Aphragmophora Tokioka, 1965
Family Sagittidae Claus and Grobben, 1905
Genus Sagitta Quoy and Gaimard, 1827
Family Pterosagittidae Tokioka, 1965
Genus Pterosagitta Costa, 1869
Family Krohnittidae Tokioka, 1965
Genus Krohnitta Ritter-Záhony, 1910

The genera Krohnittella Germain and Joubin, 1912; Bathybelos Owre, 1973; and Pterokrohnia Srinivasan 1986 need further description in order to be classified in the order Aphragmophora of the system outlined above, although Bieri (1991) created a family for each of them. Bathyspadella, known from a single specimen, was placed in the Eukrohniidae by Tokioka (1965); indeed it has glandular canals, but all other morphologic characters, and its probable benthic habitat, suggest that it may belong in the family Spadellidae.

(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)

The vernacular name arrow worm is frequently given chaetognaths because of their arrow-shaped bodies. The “chaeto” part of their name refers to their moveable hooks, with which chaetognaths grasp living prey. All arrow worms are marine predators that detect prey with vibration sensors. Copepods (Phylum Crustacea) are their principal food. Chaetognaths also consume other planktonic crustaceans and fish larvae (Phylum Craniata). Each hook or adjacent tiny tooth may pierce prey exoskeletons; paralytic neurotoxins released by the arrow worm prevent prey from escaping. These neurotoxins, recently isolated from the heads of arrow worms, paralyze by blocking sodium channels in cell mem-branes, suggesting how these carnivores can capture prey as large as themselves. The toxins are likely secreted from pores adjacent to the arrow worm mouth, confirming researcher Robert Bieri’s (Antioch College) epithet for arrow worms “cobras of the sea.”
Arrow worms range in length from 0.5 to 15.0 cm. The 70 or so arrow worm species are common plankton worldwide in open seas and near shore from Spitsbergen, Norway, to the Indo-Pacific Ocean, especially abundant in warm seas to a depth of about 200 m. In 1844, Charles Darwin observed that “between latitudes 37 degrees and 40 degrees S [off the Atlantic coast of South America], the sea, especially during the night, swarmed with them.” In the transparent chaetognaths from upper-ocean levels, food is visible in the intestine through the body wall; several blue chaetognaths—perhaps colored by their blue copepod prey—have been observed. Orange bands sometimes seen on Eukrohnia may be derived from copepod prey or microrganisms on the arrow worm’s surface. Some deep-water chaetognaths are colored brightly: red, orange, and pink; other deep-water chaetognaths are transparent. Spadella is the only benthic (bottom-dwelling) genus; with caudal papillae, it temporarily adheres to substrate and then waits for prey. Although chaetognaths cannot swim against sea currents, many migrate daily, swimming up to surface waters at night and sinking downward by day, perhaps to escape predators; these diurnal migrations vary with vertical distribution of prey and with temperature.
Fine rays of unknown composition internally support one or two pairs of lateral translucent fins and a tail fin. These free-swimming predators dart forward or backward by flapping their tails and contracting striated longitudinal muscles. Lateral fins stabilize arrow worms and do not flap. The arrow worm draws part of its cuticle-covered body wall (cephalic hood) over its spiny head between feeding and sinks gently through the ocean. This hood is believed to decrease friction while the arrow worm is swimming.
The chaetognath body cavity is divided into three compartments by a septum between head and trunk and by a septum between trunk and tail. The chaetognath outer cuticle lacks chitin, and the hooks and teeth are no longer considered chitinous. Prey is swallowed whole. Chaetognath digestion is poorly known; only starch- and glycogen-splitting enzymes have been demonstrated. No proteolytic enzymes have been demonstrated in the digestive tract. Posterior to the teeth is a vestibule leading to a ventral mouth, followed by a pharynx at the beginning of the straight intestine and ending in an anus at the trunktail septum. Solid waste is eliminated from the anus. Chaetognaths lack circulatory, respiratory, and excretory organs. Cilia circulate the fluid within each of the three body cavities, distributing nutrients and dissolved wastes. Oxygen enters directly through the body wall, and carbon dioxide leaves the same way.
The nervous system consists of a large aggregation of nerve-cell bodies called the ventral ganglion linked to a large cerebral ganglion. Nerves lead to trunk muscles, tail, gut, hooks, and eyes. Sensitive external papillae (cilia) do not sense touch but rather enable the arrow worm to detect vibrations, chemicals, or water flow or all three. The V-shaped wings of Pterosagitta draco may be receptors that can distinguish different frequencies. The hooks, tail fin, and body surface sense touch. Each of two eyes consists of five inverted pigment cup ocelli located dorsally in the head. Arrow worms sense motion and differences in light intensity, although the eyes are probably unable to form visual images.
All reproduction in arrow worms is sexual, and each worm is hermaphroditic. Ovaries along each side of the intestine in the trunk coelom produce eggs; testes in the tail coelom produce sperm. The sperm mature before the eggs; sperm are formed into a single spermatophore (sperm packet) in the seminal vesicles and then released to the outside by rupture of these vesicles. Spermatophores attach themselves to the fins of the worm that produced them or to the fins of a partner. Both self- and cross-fertilization may take place in some species. Others, such as the benthic arrow worm Spadella, cross-fertilize. Two arrow worms approach and lie side by side, facing in opposite directions; then each attaches a spermatophore to the neck of the other. The sperm stream along each arrow worm’s back and through the opening into the seminal receptacle (tube along ovaries). Fertilization takes place inside the chaetognath’s body. The arrow worm oviduct is a tube inside a tube. At the posterior end, the inner oviduct tube expands to form the seminal receptacle; here sperm are received and stored. As an egg matures, a pair of cells from the inner oviduct wall form a hollow attachment stalk to the egg. Through this hollow stalk, the sperm move from the seminal receptacle to the egg. Fertilized eggs lie between the outer and inner oviduct tubes and may reach the ocean through a temporary exit (not through the female gonopore or seminal receptacle). Zygotes are brooded in some species, deposited on the seafloor, or released into the ocean. Embryos in all chaetognaths develop in the sea into diminutive adult arrow worms. Development is direct; there are no larvae. Spadella regenerates lateral and tail fins; it is the sole arrow worm species that has been successfully raised in the laboratory from hatching to sexual maturity.
Chaetognaths are phylogenetically puzzling. Fossil evidence is currently considered unhelpful in unravelling arrow worm relationships. The pattern of embryonic development justifies grouping arrow worms with deuterostomes because, in embryos of chaetognaths, hemichordates (Phylum Hemichordata), echinoderms (Phylum Echinodermata), and chordates (Phyla Urochordata through Craniata), the blastopore site becomes the anus. Although the arrow worm adult has a peritoneum-lined coelom, the formation of the arrow worm embryonic coelom differs from that of other deuterostome coelomates; therefore the phylogenetic affinities of arrow worms are obscure. In another facet of embryonic development, however, arrow worms differ from echinoderms and hemichordates: arrow worms lack ciliated larvae. Like photoreceptors of other deuterostomes, those of chaetognaths develop from cells with undulipodia that contain peripheral double tubules but lack central tubules. An intriguing link between chaetognaths and pseudocoelomates is that nematodes (pseudocoelomates, Phylum Nematoda), tardigrades (protostomous coelomates, Phylum Tardigrada), and chaetognaths have longitudinal but no circular muscles. As Charles Darwin—the first to accurately describe the action of chaetognath spines—observed, chaetognaths are remarkable for the obscurity of their affinities.
Chaetognaths are important to marine fisheries: arrow worms are food for adult herring. However, larvae of economically valuable fish such as herring occasionally fall prey to arrow worms. Chaetognath species are distributed according to temperature; the species distributions have been used in tracing the course of ocean currents. The distribution of Sagitta bipunctata, for example, found in waters of the continental shelf off North Carolina, indicates the location of lateral extensions of the Florida Current.

Phylum Chaetognatha