Fossil range: Lower Cambrian–Recent
Chaetognatha, meaning hair-jaws, and commonly known as arrow worms, are a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.079 to 4.7 in).
There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large.
Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. They The body is divided into a distinct head, trunk, and tail. There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. All chaetognaths are carnivorous, preying on other planktonic animals.
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish. Unlike those of vertebrates, however, these are composed of a thickened basement membrane extending from the epidermis, and they are not homologous. An additional caudal fin covers the post-anal tail.  At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.
The body cavity lacks a peritoneum, and therefore resembles the pseudocoel of animals such as nematodes, but is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory, circulatory, or excretory systems.
The mouth opens into a muscular pharynx which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just in front of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end.
The nervous system is relatively simple, consisting of a ganglionated nerve ring surrounding the pharynx. The dorsal ganglion is the largest, but nerves extend from all the ganglia along the length of the body. Chaetognaths have two compound eyes, each comprised of a number of pigment-cup ocelli fused together. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the lateral line in fish. An additional, curved, band of sensory bristles lies over the head and neck.
Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus.
The arrrowworm rhabdomeres are derived from microtubules, which in turn form conical bodies, which contain granules and thread structures. The cone body is derived from a cilium, and the microtubules of the rhabdomeres are 20 nm long and 50 nm wide. 
Each animal possesses a pair of testes within the tail, and a pair of ovaries in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, and then swim through a short duct to a seminal vesicle where they are packaged into a spermatophore.
During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.
Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification. Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification. The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis - in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they may have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described. Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha ercainella Chen & Huang and Protosagitta spinosa Hu) and the Middle Cambrian Burgess Shale of British Columbia (Oesia disjuncta Walcott). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then. 
CHAETOGNATHA, the name given by R. Leuckhart to a small group of transparent and for the most part pelagic organisms, whose position in the animal kingdom is a very isolated one. Only three genera, Sagitta, Spadella and Krohnia, are recognised, and the number of species is small. Nevertheless these animals exist in extraordinary quantities, so that at certain seasons and under certain conditions the surface of the sea seems almost stiff with the incredible multitude of organisms which pervade it. Rough seas, &c., cause them to seek safety in dropping into deeper water. Deep-sea forms also occur, but in spite of this the group is essentially pelagic.
As a rule the body is some I to 2 or 3 cm. in length, though some species are larger, by 4 or 5 mm. in breadth, and it is shaped __st something like a torpedo with side flanges and a slightly swollen, rounded head. It can be divided into three regions - (i.) head, f (ii.) trunk, and (iii.) tail, separated from one another by two transverse septa. The almost spherical head is covered by a hood which can be retracted; it bears upon its side a number of sickle-shaped, chitinous hooks and one or more short rows of low 89 spines - both of these features are used in characterizing the various species. A pair of eyes lie dorsally and behind them is a b closed circlet, often pulled out into various shapes, of modified epidermis, to which an olfactory function has been attributed. The interior of the head is filled up with masses of muscle fibres which are mainly occupied with moving the sickle-shaped hooks. The .- trunk contains a spacious body-cavity filled during the breeding season by the swollen ovaries, and the same is true of the tail if we substitute testes for ovaries.
The skin consists of a transparent cuticle excreted by the underlying ectoderm, the cells of which though usually one-layered may be heaped up into several layers in the head; beneath this is a basement membrane, and then a layer of longitudinal muscle fibres which are limited inside by a layer of peritoneal cells. The muscles are striated and arranged in four quadrants, two dorso-lateral and two ventro-lateral, an arrangement which recalls that of the Nematoda, whilst in their histology they somewhat resemble the muscles of the Oligochaeta. Along each side of the body stretches a horizontal fin and a similar flange surrounds the tail. Into these fins, which are largely cuticular and strengthened by radiating bars, a single layer of ectoderm cells projects.
The mouth, a longitudinal slit, opens on to the ventral surface of the head. It leads into a straight alimentary canal whose walls consist of a layer of ciliated cells ensheathed in a thin layer of peritoneal cells. There is no armature, and no glands, and the whole tract can only be divided into an oesophagus and an intestine. The latter runs with no twists or coils straight to the anus, which is situated at the junction of the trunk with the tail. A median mesentery running dorsoventrally supports the alimentary canal and is continued behind it into the tail, thus dividing the body cavity into two lateral halves.
There are no specialized circulatory, respiratory or excretory organs.
The nervous system consists of a cerebral ganglion in the head, a conspicuous ventral ganglion in the trunk, and of lateral cornmissures uniting these ganglia on each side. The whole of this system has retained its primitive connexion with the ectoderm. The cerebral ganglion also gives off a nerve on each side to a pair of small ganglia, united by a median commissure, which have sunk into and control the muscles of the head. As in other animals there is a minute but extensive nervous plexus, which permeates the whole body and takes its origin from the chief ganglia. In addition to the eyes and the olfactory circle on the head scattered tactile papillae are found on the ectoderm.
Chaetognatha are hermaphrodite. The ovaries are attached to the side walls of the trunk region; between them and the body wall lie the two oviducts whose inner and anterior end is described as closed, their outer ends opening one on each side of the anus, where the trunk joins the tail. According to Miss N. M. Stevens the socalled oviduct acts only as a "sperm-duct" or receptaculum seminis. The spermatozoa enter it and pass through its walls and traverse a minute duct formed of two accessory cells, and finally enter the ripe ovum. Temporary oviducts are formed between the "spermduct" and the germinal epithelium at each oviposition. A number of ova ripen simultaneously. The two testes lie in the tail and are formed by lateral proliferations of the living peritoneal cells. These break off and, lying in the coelomic fluid, break up into spermatozoa. They pass out through short vasa deferentia with internal ciliated funnels, sometimes an enlargement on their course - the seminal vesicles - and a minute external pore situated on the side of the tail.
With hardly an exception the transparent eggs are laid into the sea and float on its surface. The development is direct and there is no larval stage. The segmentation is complete; one side of the hollow blastosphere invaginates and forms a gastrula. The blastopore closes, a new mouth and a new anus subsequently arising. The archenteron gives off two lateral pounchs and thus becomes trilobed. The middle lobe forms the alimentary canal; it closes behind and opens to the exterior anteriorly and so makes the mouth. The two lateral lobes contain the coelom; each separates off in front a segment which forms the head and presumably then divides again to form anteriorly the trunk, and posteriorly the tail regions. An interesting feature of the development of Chaetognaths is that, as in some insects, the cells destined to form the reproductive organs are differentiated at a very early period, being apparent even in the gastrula stage.
The great bulk of the group is pelagic, as the transparent nature of all their tissues indicates. They move by flexing their bodies. Spadella cephaloptera is, however, littoral and oviposits on sea-weed, and the "Valdivia" brought home a deep-sea species.
The three genera are differentiated as follows: - Sagitta M. Slabber, with two pairs of lateral fins. This genus was named as long ago as 1775.
Krohnia P. Langerhans, with one lateral fin on each side, extending on to the tail.
Spadella P. Langerhans, with a pair of lateral fins on the tail and a thickened ectodermic ridge running back on each side from the head to the anterior end of the fin.
The group is an isolated one and should probably be regarded as a separate phylum. It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these. The most that can be said is that the Chaetognaths begin life with three segments, a feature they share with such widelydiffering groups as the Brachiopoda, the Echinoderma and the Enteropneusta, and probably Vertebrata generally.
See O. Hertwig, Die Chaetognathen, eine Monographie (Jena, 1880); B. J. Grassi, Chetognathi: Flora u. Fauna d. Golfer von Neapel (1883); S. Strodtman, Arch. Naturg. lviii., 1892; N. M. Stevens, Zool. Jahrb. Anat. xviii., 1903, and xxi., 1905. (A. E. S.)
Classes: Archisagittoidea - Sagittoidea
| Arrow worms|
Fossil range: Lower Cambrian to Recent
About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.079 to 4.7 in).