Important divisions of body parts in balanoglossus are given below:
Body is unsegmented but divisible into three regions or parts – proboscis, collar and trunk.
(1) Proboscis:
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It is the anterior most part of the body. It has muscular wall and encloses a cavity called, proboscis coelom.
It opens outside through a minute proboscis pore situated mid-dorsally near its base. Its posterior neck forms a slender neck or proboscis stalk through which it is attached to the collar.
Below the proboscis stalk, a U shaped ciliated epidermal depression, preoral ciliary organ is situated. It tests the quality of food and water, entering the mouth.
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(2) Collar:
It is the middle short region of the body. Its flap like or funnel like anterior margin is termed collarrette, which completely covers the proboscis stalk.
Ventrally below the stalk the collarette encloses, a permanent wide aperture, the mouth. It leads into buccal cavity inside the collar.
Collar is highly muscular and encloses a cavity, the collar coelom. It opens to outside through a pair of collar pores into the first pair of gill pouches behind.
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(3) Trunk:
It is the largest and posterior part of the body. It is rather flat and appears annulated due to circular constriction on the surface.
All along its length, trunk bears a mid-dorsal and a mid-ventral ridge, each accommodating its corresponding nerve and blood vessels. It is further differentiated into three regions.
(a) Branchiogenital region:
This region of trunk is marked by a pair of lateral, thin, flat and longitudinal genital wings. Gonads are enclosed in the wings and open outside by a microscopic gonopore.
On either side of the mid-dorsal ridge, two longitudinal ridges bearing two rows of branchial aperture or gill pores are found.
Number of gill pores increases with the age of animal and can be hidden by the closing of genital wings.
(b) Hepatic region:
It is smaller middle region of the trunk and bears numerous irregular intestinal sacculations or hepatic caeca on the dorsal surface.
(c) Post hepatic region:
It is the posterior most and longest part of the trunk also called abdomen or caudal region. It is more or less uniform in diameter but its posterior end slightly tapers and bears a terminal anus.