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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Affinities and Systematic Position of Balanoglossus With non-chordate

Affinities with non-chordates :-

(B) With Pogonophora :-

(a) Similarities :-

(i) Coelom → Enterocoelic

(ii) Nervous system → Intraepidermal

(iii) Gonads → In trunk region

(iv) Pericardial sac → Present

(b) Dissimilarities :-

Features	Balanoglossus	Pogonophora
Main nervous system  concentrated at	Mesosoma	Protosoma
Coelomoducts	Absent	Present
Alimentary canal	Present	Absent

Remark :-

Young (1983) opposed the resemblances between the two groups (as stated by Barington,1965; Parker and Haswell, 1972; and Mc Ferland et al, 1985), claiming the similarities are superficial.

Affinities with non-chordates :-

(B) With Echinodermata :-

(a) Similarities :-

(i) In Adult Form :-

(i) Coelom → Divisible in 3 successive parts anterior-posteriorly.

(ii) Nervous Plexux → Subepithelial

(ii) In Larval Form :- 

Tornaria resembles Bipinaria in.

(i) Pre-oral ciliated bands → Present

(ii) Cleavage → Holoblastic & equal

(iii) Apical Plate → With a turf of cilia.

(iv) Blastopore → Become Anus in adult.

(v) Enterocoelic coelom → Present.

(b) Dissimilarities :-

(i) In Adult Form :-

Features	Balanoglossus	Echinodermata
Water vascular system	Absent	Present
Pharyngeal gill slits	Present	Absent
Stomochord	Present	Absent

(ii) In Larval Form :- 

Features	Balanoglossus	Echinoderms
Eye-spot in apical plate	Present	Absent
Telotroch	Present	Absent

Remark :-

Based on resemblances; Hemichordates & Echinoderms are thought to be originated from some common remote ancestor (Hymen,1959) and Barrington,1965). H.B fell (1963) denied it on the ground that similarities were due to larval convergence in similar mode of living.

Affinities and Systematic Position of Balanoglossus with non-chordate

Affinities with non-chordates :-

(B) With Phoronida :-

(a) Similarities :-

(i) In Adult Form :-

(i) Body → Worm like

(ii) Mode of Feeding → Ciliary

(iii) Sense Organ → Absent

(ii) In Larval Form :-

(i) Body → Spindle shaped; Transparent; Free-swimming;

(ii) Pre-oral cilia → Present

(iii) Coelomic Compartment → Five

(iv) Apical Plate → With a turf of cilia and eye spots.

(v) Blastopore → Become Anus in adult.

(b) Dissimilarities :-

(i) In Adult Form :-

Features	Balanoglossus	Pheronida
Body	Divisible into 3 parts	Not so
Pharyngeal gill slits	Present	Absent
Stomochord	Present	Absent
Sexes	Separate	Not Separate

(ii) In Larval Form :- 

Features	Tornaria	Actinotrocha
Ciliated bands	Pre-oral & Pre-anal	Along the arms
Arm like Projection	Absent	Present
Coelom formation	Enterocoelic	Schizocoelic

Remark :-

Similarities are so superficial that Balanoglosssus can not be grouped with Phoronids; as suggested by Goodrich at al.

Affinities and Systematic Position of Balanoglossus

Affinities with non-chordates :-

(A) With Annelida :-

(a) Similarities :-

(i) In Adult Form :-

(1) Body → elongated; vermiforms; coelomate;

(2) Heart → dorsal to gut.

(3) General Sense Organ → Absent

(4) Collar → Resembles to clitellum.

(5) Blood Vessel → Similar pattern.

(ii) In Larval Form :-

  

Tornaria of Balanoglossus resembles Trochophore larva of anelids in :

(1) Body → Transparent and spindle-shaped.

(2) Habit → Free-swimming

(3) Ciliated bonds → Transverse

(4) Apical Plate → With a tuft of cilia and eye spots.

(b) Dissimilarities :-

(i) In Adult Form :-

Features	Balanoglossus	Annelids
Body	Unannulated	Annulated
Pharyngeal gill slits	Present	Absent
Stomochord	Present	Absent
Excretory  organ	Glomerulus	Nephridia

(ii) In Larval Form :- 

Features	Tornaria	Trochophore
Cleavage	Equal	Spiral
Blastula	Hollow	Solid
Coelom formation	Enterocoelic	Schizocoelic

Remark :- 

Dissimilarities are much more prominent than superficial similarities. Balanoglossus, thus, could not be grouped together with Annelids.