Balanoglossus is a member of the Hemichordata, Balanoglossus have a soft body and their body have a hollow cavity, it is true coelom or body cavity. In Balanoglossus coelom is enterocoelom, during embryonic development archenteron form the body cavity or coelom, the coelom is lined by a peritoneum membrane.
Hollow body cavity spread throughout the whole body, proboscis, collar and trunk have hollow cavity and coelom in different body parts is not fused. Coelom in proboscis, collar and trunk are separated from each other and in adult Balanoglossus the coelom contains five cavities.
During embryonic development archenteron forms, five separate pouches and they form the enterocoelom in Balanoglossus. Among the five archenteron pouches, one pouch forms the body cavity in the proboscis, two pouches form the body cavity in collars and the other two pouch form the body cavity in the trunk region.
The one archenteron pouch forms proboscis coelom in proboscis region, two pouches form collar coelom in the collar region and two pouches form trunk coelom in the trunk region. In adult Balanoglossus the coelomic cavity is not very clear due to the presence of muscle fibres and connective tissues which makes the coelomic cavity irregular.
The coelomic cavity in Balanoglossus can be divided into three regions, proboscis coelom, collar coelom and trunk coelom, each part of the coelom have its special features and we will discuss all those parts of the coelom in Balanoglossus in detail.
Table of Contents
Proboscis Coelom or Protocoel:
It is the anterior-most part of the coelom, coelom in proboscis region is unpaired, coelom in proboscis region also known as protocoel.
Coelom in proboscis region is not conspicuous, the connective tissues and muscle fibres make the proboscis coelom. In the proboscis region, only a small cavity represents the proboscis coelom and most of part of proboscis coelom get damaged due to the connective tissue and muscle fibres.
The proboscis coelom is occupied by proboscis complex which includes the buccal diverticulum, glomerulus, heart vesicles. The proboscis coelom opens outside through proboscis pore present at the base of proboscis stalk mid-dorsally, the proboscis canal connects proboscis coelom with proboscis pore.
Collar Coelom or Mesocoel:
In collar region collar coelom present in pairs, two collar coelom present laterally in the collar region. In the collar region buccal cavity make a central cavity and there is a narrow space between the wall of the buccal cavity and the body wall of the collar region, mid-dorsal mesentery and mid-ventral mesentery divide the narrow canal between the buccal cavity and body wall in Balanoglossus.
Two coelomic cavities in the collar region do not open to proboscis coelom, both lateral collar coelom opens outside through collar canal and collar pore into the first gill pouches posteriorly.
Collar coelom or mesocolon is also have reduced space due to the presence of collar muscle fibres and connective tissues and making the collar coelom irregular.
Trunk Coelom or Metacoelom:
The coelom is the trunk region also presents in pairs, one of each present laterally in the trunk region. Between the wall of the gut and trunk body wall have a narrow space that forms the trunk coelom or metacoel.
The coelom does not communicate with the collar coelom because a collar-trunk septum separates the collar coelom from the trunk coelom. Two lateral trunk coelom is separated from each other by incomplete mid-dorsal mesentery and complete mid-ventral mesentery.
Each trunk coelom is again divided into two parts, the dorsolateral compartment and the ventrolateral compartment by a lateral septum. Like other parts of the coelom, trunk coelom also become irregular in adult Balanoglossus due to the growth of muscle fibres and connective tissue in the trunk region.
Trunk coelom is completely closed they are not opened outside like collar coelom and proboscis coelom.
Coelomic Fluid of Balanoglossus:
In the collar region and proboscis region the body cavity or coelom is not closed, they open outside through coelomic pore so they are not filled by body fluids, collar coelom and proboscis coelom filled with seawater.
But the trunk coelom is closed and functions like a typical body cavity, trunk coelom is filled with coelomic fluid. In trunk coelom, coelomic fluid has a special type of ameboid cells coelomocytes.
Coelomocytes originate from the epithelial cells of the coelom and some scientists believe that the coelomocytes have similar functions as WBC, coelomocytes gives protection against foreign harmful microorganisms.
Reference:
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