The Phylum Chaetognatha
Etymology: From the Greek Chaite for long hair, and gnathos for jaw.
Characteristics of Chaetognatha:
- Bilaterally symmetrical and vermiform.
- Body has more than two cell layers, tissues and organs.
- Body cavity a pseudocoelom
- Body possesses a through gut with a non-terminal anus.
- Body divided into three sections, a head, a trunk and a tail.
- Nervous system is a circum-pharangeal ganglionated ring.
- Has a no circulatory system or gaseous exchange organs.
- Possesses no excretory system.
- Reproduction normally sexual and hermaphroditic.
- Feeds on fine particules in the water.
- All live marine environments.
The Chaetognatha (Arrow Worms) are a small – just over 100 species have been named – though unusual group of animals, which appear not to be to closely related to any other phyla.
They are small, long, thin and possess side fins and and tail fins. They swim by means of their tail fins, the side fins being used only for stabilization. They are an old group of animals with fossils known from over 300 million years ago.
They are all marine, and most species are planktonic, living in the open ocean. A few species are benthic, preferring the sea bottom and Spadella cephaloptera can be found in rock pools on many European coastlines. Many species are associated with particular water currents in the oceans. For instance, the edges of the Florida Current on the continental shelf of North Carolina can be detected by the presence of absence of Sagitta bipunctata.
Like many other members of the planktonic world, some chaetognaths migrate up and down in the water column. They come to the surface to feed at night then retire to deeper waters as the sun rises. Unlike other planktonic organisms, some chaetognaths migrate annually – living in the surface waters during the winter but retiring to deeper waters in summer.
The planktonic species occur in large numbers in warmer oceans where they are voracious predators on other species of plankton such as copepods. They may in fact be the primary predator in many planktonic environments. Larger species may reach 10cm (4 inches) or more in length and are known to attack small fish. They have a series of movable spines on their head which they use to grasp and hold their prey while it is eaten. They have a strange hood, part of their body wall, which they can drag over the spines and the rest of their head when they are not eating.
Chaetognaths have two compound eyes, each made up of 5 ocelli. It is doubtful if they can see very much, but they can certainly detect changes in light intensity.
All known Chaetognaths are hermaphrodites, meaning they possess both male and female sex organs. The female organs, the ovaries, are just behind the mid-body mark while the male sexual organs, the testis, are in the tail. The sperm mature before the ova (eggs) which may help avoid self fertilisation to some extent. However self fertilisation is known to occur.
In Spadella cephaloptera,pairs of Chaetognaths may lie beside each other head to tail such that they fertilise each other.
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