Phylum Chaetognatha
Phylum
Chaetognatha
A common name for chaetognaths is arrowworms. They are all marine animals and are highly specialized for their planktonic existence. Their relationship to other groups is obscure, although embryological characters indicate deuterostome affinities.
The name Chaetognatha (ketog ´na-tha) (Gr. chaite, long flowing hair, + gnathos, jaw) refers to the sickle-shaped bristles on each side of the mouth. This is not a large group, for only some 65 species are known. Their small, straight bodies resemble miniature torpedoes, or darts, ranging from 2.5 to 10 cm in length.
Arrowworms are all adapted for a planktonic existence, except for Spadella (Gr. spadix, palm frond, + ella, dim. suffix), a benthic genus. They usually swim to the surface at night and descend during the day. Much of the time they drift passively, but they can dart forward in swift spurts, using the caudal fin and longitudinal muscles— a fact that no doubt contributes to their success as planktonic predators. Horizontal fins bordering the trunk are used in flotation rather than in active swimming.
Form and Function
The body of an arrowworm is unsegmented and includes a head, trunk, and postanal tail (Figure 24-1A). On the underside of the head is a large vestibule leading to the mouth. The vestibule contains teeth and is flanked on both sides by curved chitinous spines used in seizing prey. A pair of eyes is on the dorsal side. A peculiar hood formed from a fold of the neck can be drawn forward over the head and spines. When the animal captures prey, it retracts the hood, and the teeth and raptorial spines spread apart and then snap shut with startling speed. Arrowworms are voracious feeders, living on planktonic forms, especially copepods, and even small fish (Figure 24-1B). When they are abundant, as they often are, they may have a substantial ecological impact. They are nearly transparent, a characteristic of adaptive value in their role as planktonic predators.
A thin cuticle covers the body, and the epidermis is single layered except along the sides of the body, where it is stratified in a thick layer. These are the only invertebrates with a many-layered epidermis.
Arrowworms have a complete digestive system, a well-developed coelom, and a nervous system with a nerve ring containing large dorsal and ventral ganglia and a number of lateral ganglia. Sense organs include eyes, sensory bristles, and a unique U-shaped ciliary loop that extends over the neck from the back of the head. The ciliary loop may detect water currents or may be chemosensory. However, vascular, respiratory, and excretory systems are entirely lacking.
Arrowworms are hermaphroditic with either cross- or self-fertilization. Eggs of Sagitta (L. arrow) bear a coat of jelly and are planktonic. Eggs of other arrowworms may be attached to the body and carried about for a time. Juveniles develop directly without metamorphosis. Chaetognath embryogenesis differs from that of typical deuterostomes in that the coelom is formed by a backward extension from the archenteron rather than by pinched-off coelomic sacs. There is no true peritoneum lining the coelom. Cleavage is radial, complete, and equal.
A common arrowworm is Sagitta (Figure 24-1A).
Position in Animal Kingdom
A common name for chaetognaths is arrowworms. They are all marine animals and are highly specialized for their planktonic existence. Their relationship to other groups is obscure, although embryological characters indicate deuterostome affinities.
The name Chaetognatha (ketog ´na-tha) (Gr. chaite, long flowing hair, + gnathos, jaw) refers to the sickle-shaped bristles on each side of the mouth. This is not a large group, for only some 65 species are known. Their small, straight bodies resemble miniature torpedoes, or darts, ranging from 2.5 to 10 cm in length.
Arrowworms are all adapted for a planktonic existence, except for Spadella (Gr. spadix, palm frond, + ella, dim. suffix), a benthic genus. They usually swim to the surface at night and descend during the day. Much of the time they drift passively, but they can dart forward in swift spurts, using the caudal fin and longitudinal muscles— a fact that no doubt contributes to their success as planktonic predators. Horizontal fins bordering the trunk are used in flotation rather than in active swimming.
Form and Function
The body of an arrowworm is unsegmented and includes a head, trunk, and postanal tail (Figure 24-1A). On the underside of the head is a large vestibule leading to the mouth. The vestibule contains teeth and is flanked on both sides by curved chitinous spines used in seizing prey. A pair of eyes is on the dorsal side. A peculiar hood formed from a fold of the neck can be drawn forward over the head and spines. When the animal captures prey, it retracts the hood, and the teeth and raptorial spines spread apart and then snap shut with startling speed. Arrowworms are voracious feeders, living on planktonic forms, especially copepods, and even small fish (Figure 24-1B). When they are abundant, as they often are, they may have a substantial ecological impact. They are nearly transparent, a characteristic of adaptive value in their role as planktonic predators.
A thin cuticle covers the body, and the epidermis is single layered except along the sides of the body, where it is stratified in a thick layer. These are the only invertebrates with a many-layered epidermis.
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| Figure 24-1 Arrowworms. A, Internal structure of Sagitta. B, Scanning electron micrograph of a juvenile arrowworm, Flaccisagitta hexaptera (35 mm length) eating a larval fish. |
Arrowworms have a complete digestive system, a well-developed coelom, and a nervous system with a nerve ring containing large dorsal and ventral ganglia and a number of lateral ganglia. Sense organs include eyes, sensory bristles, and a unique U-shaped ciliary loop that extends over the neck from the back of the head. The ciliary loop may detect water currents or may be chemosensory. However, vascular, respiratory, and excretory systems are entirely lacking.
Arrowworms are hermaphroditic with either cross- or self-fertilization. Eggs of Sagitta (L. arrow) bear a coat of jelly and are planktonic. Eggs of other arrowworms may be attached to the body and carried about for a time. Juveniles develop directly without metamorphosis. Chaetognath embryogenesis differs from that of typical deuterostomes in that the coelom is formed by a backward extension from the archenteron rather than by pinched-off coelomic sacs. There is no true peritoneum lining the coelom. Cleavage is radial, complete, and equal.
A common arrowworm is Sagitta (Figure 24-1A).
Position in Animal Kingdom
- Hemichordates belong to the deuterostome branch of the animal kingdom and are enterocoelous coelomates with radial cleavage.
- A chordate plan of structure is suggested by gill slits and a restricted dorsal tubular nerve cord.
- Similarity to echinoderms is shown in larval characteristics.
- A tubular dorsal nerve cord in the collar zone may represent an early stage of the condition in chordates; a diffused net of nerve cells is similar to the uncentralized, subepithelial plexus of echinoderms.
- Gill slits in the pharynx, which are also characteristic of chordates, serve primarily for filter feeding and only secondarily for breathing and are thus comparable to those in protochordates.
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