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.