In this article we will discuss about the gradual appearance of chordate characters in protochordates .
The chordates exhibit different gradations of structural organisation from simple to complex. The protochordates including the three subphyla—Hemichordata, Urochordata and Cephalochordata—are the primitive members of the Phylum Chordata. All of them lack vertebral column and skull. They form an intermediate group between the non-chordates and vertebrates.
Consideration of Hemichordata:
The Phylum Hemichordata represents the most primitive group which shows many resemblances with the non-chordates and show the least number of chordate characters.
The number of non-chordate characters in Hemichordata out-weights the number of chordate characters; e.g., they have;
(i) Annelid-like vermiform body and similar disposition and direction of flow of blood,
(ii) Echinoderm-like intra-epidermal nervous system,
(iii) A peculiar tornaria larva which resembles the trochophore larva of annelids on one hand and the echinoderm larvae on the other,
(iv) No respiratory pigment and
(v) The presence of arginine phosphate which is not seen in any higher chordate.
The inclination towards chordate nature is very feebly marked.
Among the minor resemblances between them and the chordates are:
(i) Holoblastic cleavage and enterocoelous coelom. But these are not unique for chordates only, because these are common features of all Deuterostomia.
(ii) Creatine phosphate is present as in higher chordates. But this point also is not very important. Creatine phosphate is present in many echinoderms as well,
(iii) They show ciliary mode of feeding like other protochordates. But this point is negligible, as food-collection is largely external in them whereas it is totally internal in other protochordates.
Now, we consider the status of the three unique chordate characters in Hemichordata. At first there is no homologue of the notochord in them.
The resemblance between the nervous system of Balanoglossus and the chordates is lying on a better strand. Discussion on it has been made along with the justification of the statement “whether Balanoglossus is a forerunner of chordates or not”.
The only resemblance to chordate organisation lies in the presence of pharyngeal gill-slits. The branchial apparatus structurally resembles that of other protochordates.
It may thus be suggested that the organisation of hemichordates is very little ahead from that of echinoderms and they can be said to incline to chordate organisation only due to the presence of pharyngotremy and a partial resemblance between the nervous system in them and chordates.
Undoubtedly the number of chordate features in hemichordates is much lesser than protochordates. Actually the hemichordates are an early offshoot from the main line of chordate evolution.
Due to their close resemblance with echinoderms and very little resemblance with chordates, many workers treat Hemichordata as a separate phylum in between Echinodermata and Chordata and many workers place it as the first subphylum under Phylum Chordata.
Consideration of Urochordata:
The urochordates on the other hand show an extreme degenerated organisation in the adult stage. The adult urochordates show a few non-chordate characters.
The non-chordate features of urochordates are:
i. Hermaphrodite nature,
ii. Alternation of generation in many,
iii. Typhlosole in intestine,
iv. Asexual reproduction by budding in many,
v. No respiratory pigment present, and a few others.
The chordate characters are not very pronounced in adults, except the following features:
i. Internal food-collection which undoubtedly is an advancement over Hemichordata,
ii. Pharyngeal gill-slits,
iii. Floor of pharynx supported by endostyle which is thought to be homologous with thyroid gland,
iv. Atrial component like cephalochordates,
v. Neural gland is thought to be homologous to the hypophysis of vertebrates,
vi. Velum guarding the mouth as in ammocoetes larva of cyclostomes. But this character in ammocoetes larva is merely a specialised feature for specialised type of feeding and protection of highly extensive pharynx.
However, the tadpole larvae show a number of chordate characters like:
i. Notochord, though restricted at caudal region only,
ii. Dorsal tubular nerve cord,
iii. Segmental muscle bands, though restricted at caudal end,
iv. Heart with epicardia, though nonfunctional,
v. Tail-fin continuous with dorsal and ventral fins with striae. These striae are compared with fin-rays of fishes and
vi. Holoblastic cleavage and enterocoelous coelom. These characters, however, are common for all Deuterostomia.
Due to the highly organised chordate nature of tadpole larvae, Berrill (1955) considered that the tadpole larvae in urochordates had been evolved within the group to fulfill certain specific ascidian needs and that had never been inherited from others.
He also suggested that some free-swimming tadpole larvae—by way of neoteny—gave origin to the free-swimming vertebrates. Recently, however, the urochordates are held to be a side branch from the main line of chordate evolution.
Consideration of Cephalochordata:
The cephalochordates on the other hand show the least number of non-chordate characters and maximum number of chordate characters as contrasted against other groups of protochordates.
The non-chordate characters are:
i. Protonephridia with solenocytes like annelids,
ii. External marks of segmentation,
iii. No respiratory pigment and
iv. Segmentally arranged gonads without gonoducts.
Cephalochordata represent a fair theoretical picture of chordate organisation.
Because these subphylum members possess:
ii. Dorsal tubular nerve cord,
iii. Pharyngeal gill-slits,
v. Ramification of blood vessels into hepatic diverticulum may be considered as a prophecy of hepatic portal system of higher chordates,
vi. Hepatic diverticulum is considered as a prophecy of vertebrate liver,
vii. Endostyle is shown by auto radiographic experiments to be homologous with thyroid gland,
viii. Creatine phosphate is present,
ix. Meta-pleural folds are thought to be the forerunner of paired fins of fishes,
x. Velum encountered in ammocoetes larva of cyclostomes,
xi. Hatschek’s pit is compared with the hypophysis of vertebrates,
xii. Zymogen cells as in vertebrate pancreas are present in the walls of hepatic diverticulum and
xiii. Reissner’s fibres in all vertebrates arise from the ependymal cells of the diencephalon. These fibres are also found in Branchiostoma where these fibres arise from infundibular organ. Thus infundibular organ is compared with diencephalon as both give rise to Reissner’s fibres.
Due to striking resemblances between the ammocoetes larvae of cyclostomes and Branchiostoma, many early workers claimed that Branchiostoma was permanent neotenous form of some ammocoetes larva. However, this contention is not proved to be true.
Moreover, Branchiostoma cannot be regarded as the direct ancestor of vertebrates and other chordates due to its possession of many specialised as well as degenerated features like
i. Sedentary life,
ii. Reduced brain and sense organs probably in accompaniment of the sessile life,
iii. The anterior projection of notochord into the head far beyond the brain,
iv. Presence of velum, atrium, oral hood, buccal cirri, very large number of gill- slits in association with specialised mode of feeding. Recently they are regarded as an offshoot from the main line of chordate evolution.
Gradual progression in the possession of chordate features from the Hemichordata to Urochordata, and Cephalochordata is observed in the evolutionary dynamics. However, even those protochordates which possess a number of chordate characters differ remarkably from the higher chordates in several embryological aspects.