In this article we will discuss about the characters and affinities of Branchiostoma.
Characters of Branchiostoma:
General Characters of Branchiostoma:
1. Marine, widely distributed in shallow waters.
2. Mostly sedentary and buried with only anterior body end, projecting above bottom sand.
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3. Body small, 5 to 8 cm long, slender, fish-like, segmented and transparent.
4. Body has trunk and tail. Head lacking.
5. Paired appendages lacking. Median fins present.
6. Exoskeleton absent. Epidermis single-layered.
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7. Muscles dorso-lateral, segmented into myotomes.
8. Coelom enterocoelous, reduced in the pharyngeal region by development of atrial cavity.
9. Notochord rod-like, persistent, extending from rostrum to tail, hence, the name Cephalochordata.
10. Digestive tract complete. Pharynx large, perforated by numerous persistent gill-slits opening into atrium.
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11. Respiration through general body surface. Special organs for respiration absent.
12. Circulatory system well developed, closed without heart and respiratory pigment. Hepatic portal system developed.
13. Excretion by protonephridia with solenocytes.
14. Nerve cord dorsal, tubular without ganglia and brain. Dorsal and ventral nerve roots separate.
15. Sexes separate. About 25 pairs of gonads. Gonoducts lacking. No asexual reproduction.
16. Fertilisation external in sea water.
17. Development indirect, including a free-swimming larva.
18. Cephalochordata has about 30 species of the genus Branchiostoma and all put in the class Leptocardii.
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Primitive, Degenerate and Specialised Characters of Branchiostoma (Cephalochordata):
Primitive Characters:
Primitive characters are relics from its original ancestors. They are those characters which are developed and perfected in the course of evolution and they generally show progress.
Primitive characters of Branchiostoma are as follows:
1. There is no specialised head.
2. Absence of paired limbs or fins.
3. The epidermis is made of single row of cells and a true dermis is absent.
4. Coelom enterocoelous.
5. A complete notochord is persistent throughout life with no vertebral column.
6. There is a complete metameric segmentation of myotomes from one end to the other, as in the embryo of vertebrates, and this obvious metamerism is retained throughout life.
7. Alimentary canal is a straight tube without loops, and the midgut diverticulum is a hollow tube. Jaws absent.
8. A peculiar pharynx specialised for ciliary feeding in which sorting of food takes place after a food-laden current of water enters the pharynx, this involves taking a very large volume of water and its continuous removal through numerous gill-clefts.
9. Blood vascular system is diagrammatically vertebrate-like but without a heart and practically no distinction between arteries and veins.
10. Special respiratory organs and respiratory pigment absent.
11. Excretory organs are segmental protonephridia having solenocytes. Protonephridia are not coelomoducts.
12. The neural tube does not form an anterior brain. The dorsal and ventral nerve roots do not unite to form spinal nerves. Dorsal roots have no ganglia, consequently impulses pass directly from the skin to the neural tube.
13. Paired sense organs such as eyes, ears and nose are absent. The receptors present are of a primitive type.
14. Gonads segmentally arranged and without gonoducts.
15. Eggs are small with almost no yolk. Blastula is hollow, spherical and single-layered.
Degenerate Characters:
Branchiostoma may be a specialised and degenerate descendant of Agnatha.
The degenerate characters include:
1. Poorly developed brain (cerebral vesicles) and simple sensory organs.
2. Lack of any cartilaginous or bony endoskeleton.
3. Lack of gonoducts.
Specialised or Secondary Characters:
The following specialised or secondary characters present in Branchiostoma are peculiar adaptations to its mode of life. They also probably prevented them to undergo further evolution. Their degenerate and specialised characters do not consider Branchiostoma in direct line of chordate evolution. Cephalochordates arose as a side offshoot.
1. There is an oral hood enclosing the mouth which is provided with oral cirri bearing sensory organs.
2. Elaborate velum with sensory velar tentacles.
3. An extreme anterior extension of notochord into rostrum seen in no other chordate making it stronger for burrowing.
4. A very large and elaborate pharynx with an accompanying multiplication of ciliated gill-clefts. The extra number of gill-clefts is unrelated to body segmentation.
5. Wheel organ and Hatscheck’s groove and pit develop which help in ciliary feeding.
6. Formation of a large atrium with an atriopore.
7. Peculiar reduction and distortion of coelom by the atrium.
8. Curious asymmetries seen in the adult and in early development.
Branchiostoma has a simple organisation compared to vertebrates because many important craniate structures are lacking in it. But it is definitely a simple chordate having a large number of primitive characters, such as notochord, dorsal hollow nerve cord, and gill-clefts.
Moreover, it has a post-anal tail, hepatic diverticulum, hepatic portal system, and the beginning of a ventral heart. These characters show the probable structure of the original chordate ancestor to which Branchiostoma is related. However, its secondary characters show that it is not on the direct line of chordate descent in evolution.
It is possible that Branchiostoma and its allies resemble in many fundamental respects the fish-like creature that existed in Silurian seas, and it is possible that the modem fishes of today evolved from animals of much the same kind.
Affinities of Branchiostoma:
Branchiostoma show affinities with non-chordate as well as chordates. Cephalochordates have been regarded to be phylogenetically related to several non-chordate groups at one time or other. But these can be overlooked in favour of the more chordate-like characteristics of cephalochordates.
Chordate Affinities:
Cephalochordata (Branchiostoma) exhibits the three basic chordate features such as notochord, dorsal tubular nerve cord and pharyngeal gill-slits in the most typical manner and there is no doubt about its chordate nature. However, it exhibits relationships with all the major groups of phylum chordata, and its real status in the phylum remains uncertain.
Affinities with Hemichordata:
Hemichordata and Cephalochordata resemble in having similar:
(i) Pharyngeal apparatus with numerous gill-slits and gill-bars,
(ii) Filter feeding mechanism,
(iii) Respiratory mechanism,
(iv) Enterocoelous coelom, and
(v) Numerous gonads without gonoducts.
Objections:
But, muscles in Hemichordata are unsegmented, nervous system distinctly of non-chordate type, gill-slits dorsal in position instead of lateral, and a post-anal tail is lacking. Moreover, inclusion of Hemichordata under chordata is also uncertain because of doubtful nature of notochord. This shows Hemichordata is more primitive than Cephalochordata.
Affinities with Urochordata:
Branchiostoma (Cephalochordata) and Herdmania (Urochordata) are regarded to be very closely related because of:
(i) The primitive ciliary feeding and respiratory mechanisms,
(ii) A large pharynx having many gill-clefts (stigmata), an endostyle which produces mucus, peripharyngeal bands and an epipharyngeal groove (dorsal lamina),
(iii) An ectoderm-lined atrial cavity opening to the exterior by an atriopore or its homologue, the atrial siphon,
(iv) The mantle of Herdmania is equivalent to the atrial folds of Branchiostoma. Besides the homologies of adult anatomy, there are embryonic similarities.
(v) There is a very striking similarity of the eggs of some tunicates to those of Branchiostoma, in both there is an identical arrangement of future parts in the cytoplasm, and
(vi) The early stages of development, especially in the formation of nervous system and notochord are practically identical.
Such vast similarities afford convincing evidence of a close relationship between Cephalochordata and Urochordata and they show that both have been derived from a common ancestor.
Objections:
The adult Urochordata are extremely degenerate, sedentary animals having several characters not found in Branchiostoma.
For Example:
(i) Body unsegmented and covered by a cellulose test,
(ii) Without hollow nerve cord and notochord,
(iii) With liver,
(iv) With a heart covered by peritoneum,
(v) Absence of nephridia,
(vi) Sexes united and gonads hermaphrodite, and
(vii) Larva undergoes retrogressive metamorphosis.
These differences show that in spite of close similarities reflecting probable common ancestry, the cephalochordates are better evolved than the urochordates.
Affinities with Cyclostomata:
Branchiostoma and ammocoete larva of lamprey (Cyclostomata) are similar in many characters such as:
(i) Elongated, slender fish-like body,
(ii) Continuous dorsal median fin,
(iii) Mouth surrounded by an oral hood,
(iv) Presence of a velum,
(v) Pharynx with an endostyle and gill-slits, and
(vi) Presence of dorsal hollow nerve cord and notochord.
Besides these fundamental chordate characters, their adults have persistent velum, gill-slits, metameric myotomes and a post-anal tail.
Affinities with Other Vertebrates:
Branchiostoma also show similarities with other vertebrates in several ways such as:
(i) Post-anal tail,
(ii) Metameric myotomes,
(iii) Coelom lined by mesodermal epithelium,
(iv) Mid-gut diverticulum comparable with liver,
(v) Well-formed hepatic portal system, and
(vi) With forward flow of blood in ventral and backward flow of blood in dorsal longitudinal blood vessels.
Objections:
Cephalochordates differ from cyclostomes and other vertebrates in most of their primitive characters, such as:
(i) Absence of head, paired limbs, skull, vertebral column, muscular true heart, blood without RBCs, brain, sense organs and gonoducts, etc., and
(ii) In possessing nephridia, atrium, numerous gonads, asymmetry, etc.