In this article we will discuss about Phylum Chaetognatha:- 1. History of Phylum Chaetognatha 2. Features of Phylum Chaetognatha 3. Affinities.
History of Phylum Chaetognatha:
1. Slabber, in the year 1769, was the first worker to record Sagitta and he placed it in the order Intestina under phylum Vermis.
2. Since then extensive works on various aspects of Sagitta have been performed by a host of workers. It was Leuckart (1854) who was the pioneer to create a separate group, Chaetognathi for Sagitta.
3. Both Leuckart and Gegenbaur (1854) placed Chaetognathi between Nematodes and Annelids.
4. Butschli (1910) placed the Chaetognathi along with Ectoprocta, Phoronida, Brachiopoda and others under the subphylum Oligomera.
5. This subphylum was in turn placed under phylum Vermis.
6. Up to the present day the relationships of Chaetognathi remain controversial. So a separate phylum is created to accommodate the different Chaetognaths.
7. The original spelling of the phylum ‘Chaetognathi’ by Leuckart is slightly modified as phylum Chaetognatha.
Besides those authors, the contributions made by Kowalevsky (1870), Hertwig (1880), Grassi (1883), Gourret (1884), Doncaster (1902), Gunther (1819), Burfield (1927), etc. are considered valuable.
Features of Phylum Chaetognatha:
1. Phylum Chaetognatha are marine planktonic animals live from the intertidal zone to several hundred metres.
2. Phylum Chaetognatha look like transparent arrow-like, hence are called arrow worms.
3. Elongated, torpedo-shaped bilaterally symmetrical animal.
4. Body of Phylum Chaetognatha is divisible into head, thorax and tail.
5. Head of Phylum Chaetognatha bears four rows of teeth and a pair of vestibular organs.
6. There are 4-14 grasping spines on the lateral side of the mouth which help in the capture of prey.
7. The trunk is provided with thin and transparent lateral fins and the tail with a caudal fin—help in locomotion.
8. Coelom formation is enterocoelic.
9. There are no circulatory, respiratory and excretory systems in Phylum Chaetognatha.
10. Nervous system is represented by brain and elongated sub-enteric and ventral ganglion.
11. They are hermaphrodites.
12. Fertilization is internal in Phylum Chaetognatha.
13. Cleavage equal and coeloblastic blastula.
14. Development direct.
Examples:
Sagitta, Spadella
Affinities of Phylum Chaetognatha:
The Chaetognaths were first discovered by Slabber in 1769. But uptil now the systematic position of Chaetognaths remains problematic. Because of structural resemblances with different invertebrates, they were included under Mollusca, Annelida, Nematoda, Arthropoda and some enterocoelous forms.
The relationship of Chaetognaths with Molluscs, Annelida and Arthropods becomes extremely difficult to establish. All the similarities are superficial in nature and have no specific phylogenetic significance.
Relationship of Phylum Chaetognatha with Nematodes:
The anatomical organisation of the Chaetognaths exhibits structural similarities with pseudocoelomate forms, specially the Nematodes.
Some such similar features are mentioned below:
(1) The fins of Chaetognaths can be compared with the alae of nematodes.
(2) Presence of ganglionated circumenteric nerve ring in both.
(3) The muscular pharyngeal bulb of Chaetognaths resembles closely to that of Nematodes.
(4) Restriction of the body wall musculature into four longitudinal muscle bands in Chaetognaths resembles that of Nematodes.
(5) Similar histological construction of muscles in both.
(6) The adhesive bristles in the head of some marine Nematodes correspond to the grasping spines of Chaetognaths.
Remarks:
Despite these similarities in adult condition, the embryological stages offer wide diversities, particularly in the development of coelom. The enterocoelous development of coelom in Chaetognaths stands as a strong barrier to establish the nematode affinity, although it has been advocated by Schneider (1866) and Metschnikov (1867).
Relationship of Phylum Chaetognatha with Brachiopoda:
Many authors, specially Lameere (1931) tried to relate Chaetognaths with the Brachiopods.
Lameere advances the following arguments to support his contention:
(1) The grasping spines of Chaetognaths are comparable to the lophophore of Brachiopoda.
(2) Existence of few similarities in embryonic development.
Remarks:
But the above points are not sufficiently convincing to establish such a relationship. The point of grasping spines seems rather superficial in nature. The development of coelom also offers points of difference. In Brachiopoda the coelom develops in four different parts of the body. So none of these two points are sufficient enough to establish relationship with Brachiopoda.
Relationship of Phylum Chaetognatha with Echinodermata and Hemichordata:
1. The enterocoelous formation of coelom in the embryonic stage of Chaetognaths led many zoologists to establish the phylogenetic relationship with the Echinodermata and Hemichordata. But the mode of formation of coelom is different in these groups.
2. In Chaetognaths coelom develops as folds of the archenteron, but in Echinoderms and Hemichordates it develops as out-pouching of archenteron.
3. Echinoderms and Hemichordates have free-swimming larval forms, but in Chaetognaths, development is direct, i.e., larval form is absent.
4. The primary division of coelom is also different in them.
Remarks:
For this reason the affinities of Chaetognaths with Echinoderms and Hemichordates cannot be established.