Affinities and Systematic Position of Balanoglossus with Chordate

Affinities with chordates :-

(a) Similarities :-

(i) Pharyngotremy (pharyngeal gill-slits) → Present

(ii) Central Nervous System → Comparable to dorsal tubular nerve cord.

(iii) Stomochord → Comparable to Notochord.

(iv) Enterocoelic coelom → Present.

(b) Objection :-

Features	Balanoglossus	Chordates
Notochord	Not true	True
Morphology	Short, hollow, stifftube	Solid, rod like
Composed of	Tall vacuolated endodermal cell	Arranged  vacuole mesodermal cells
Origin	Endodermal as a buccal diverticulam	Mesodermal
Surrounded by	Besement membrane	Primary & secondary sheathes
Position	Ventral to dorsal blood vessel	Dorsal to dorsal blood vessel
Supporting Function	Absent	Present

Remark :-

(1) Barrington and Hyman called the buccal diverticulam as stomochord instead of notochord. They also rejected the idea of persistant notochord in Balanoglossus.

(2) Central Nervous System :- More similar to invertebrate type (Barrington,1965) than any true chordates, as :

(i) Nerve cord- Intraepidermal.

(ii) Nerve ring- Circumcentric.

(iii) Ventral nerve cord- Solid

(3) Pharyngotremy :- The only Chordates feature might be a convergent feature, as conceived by Beril (1955).

(4) Other Features :- 

(i) Post anal tail- Absent

(ii) Vascular System- Without centralised heart.

(iii) Endostyle- Absent

(iv) Gut- Undifferentiated, without muscular wall, no peristasis.

Remark :-

Resemblances are superficial and such when due to evolutionary convergence in the same track. Features indicate Hemichordates are definition lower in rank than true Chordates ( Uro- & Cephalo-chordates). Hemichordates thus very remotely connected with the central stock from which true chordates descended.

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