1. The acorn or tongue worm
Balanoglossus

2. Natural history
Balanoglossus is marine and inhabits the shallow coastal waters all over the world.
Balanoglossus lives in burrows made in sand or mud . The burrow is usually U- shaped.

3. External characters Balanoglossus is elongated,cylindrical,
worm like animal.
its body is flaccid and fragile so that it is difficult to collect the larger species intact.
The skin is soft and coated with shiny mucus.
Balanoglossus gigas is over 1.5m longs and makes burrows 3m long.
The body is often brightly coloured and emits a unpleasant smell like iodoform .
The smell gives protection from prey .
The body is is differentiated into 3 regions :proboscis ,collar ,and trunk or metasome .

4. proboscis :- it is anterior part of body it is short and conical in form, tampering anteriorly . It has a thick ,muscullar and enclose cavity.
Collar:-the collar is the short middle portion of the body . It has thick ,muscullar wall that encloses a cavity ,the collar coelom.
Trunk :-it is the posterior and largest part of body . it further shows three regions :
Branchiogenital.
Middle hepatic
Posterior abdominal.

5. Locomotion
It moves with the help of cilia that cover the greater part of the body .
The ciliary action is, however ,assited by peristaltic contractions of the proboscis.
After proboscis advances and anchors , the collar and trunk are pulled forward, and the cycle is repeated again and again .

6. Burrowing
Balanoglossus burrows by means of proboscis and collar .these parts are stiffened by taking sea water into their cavities through the pores on the dorsal surface and then pushed into the sand or mud . This is aided by peristaltic contractions of the trunk.

7. Regeneration
It has good power of regeneration. Posterior part of the trunk , if cut off, is readily regrown. Anterior part, including proboscis, colar and branchiogenital region, can also be reformed.

8. Body wall
The body wall of balanoglossus consists of following parts :- epidermis, nervous layer basement membrane and musculature.
Peritoneum is lacking .
There is no cuticle.

9. Coelom
It is tricoelomate. Its coelom consists of three sets of separate cavities that arise as independent pouches from the archenteron of are separated by dorsal and ventral mesenteries, but communicate with one another in certain regions

10. Digestive system
The alimentary canal of balanoglossusis straight and complete. It is ciliated and supported by dorsal and ventral mesenteries throughout its length . It comprises the mouth, buccal cavity, pharynx , oesophagus , intestine and anus .

11. Respiratory system It possessses a gill appara-
-tus for respiration .this is
Most cordate like structure.
It includes a dorsal heart,
two main longitudinal vessels
:a dorsal and a ventral ,
interconnected by small
lateral vessels and sinuses.
Blood is colorless and with-
-out corpuscles.

12. Nervous system
Nervous system is diffused ,consisting of an epidermal plexus of nerve cells and nerve fibers .
The plexus is thickened to form two longitudinal nerve cords: a dorsal and a ventral, connected together by a circumentric nerve ring in the collar. The dorsal nerve cord may be transformed into a tubular and free collar cord in the collar. The collar cord foreshadow the future condition in chordates.

13. Excretory system
Excretory system comprises a prodoscis gland, or glomerulus situated in the proboscis and connected with blood vessels .

14. Reproduction
Sexes may be separate or united .the gonads may be several pairs or only in one pair . Fertilization is external or internal .
Asexual reproduction by budding from stolon or adhesive disc, and by fragmentation or fission of adhesive disc occurs in one group .

15. Development
Development may include a free swimming larval stage .

16. Affinities with chordates/balanoglossus
(A) Resemblances :- the hemichordates resemble the chordates in having all the three primary chordate structures , namely , gill slits, notochord and dorsal hollow nerve cord .
(B) Differences :- none of the cordates structure occurs in typical condition in the hemicordates , for instance.
The gill slits of hemicordates are dorsal in position instead of lateral as in chordates.