Division of Body: The body is covered by the test and is divisible into two parts, body proper and foot. (1) Body Proper: The distal free part of the animal is body proper. Two short cylindrical projections called the branchial and atrial siphons or funnels arise from the anterior and the dorsal end respectively.
Both the siphons are situated at the same level. The branchial or oral siphon is smaller, about 1 cm long and is directed outwards. It bears a terminal aperture or opening called the branchial aperture or mouth or incurrent opening. The terminal aperture of atrial siphon is atrial aperture or excurrent opening. Each aperture is guarded by four distinct lobes or lips formed by the much elastic test. At the slightest disturbance they contract and close the aperture.
(2) Foot: The basal attached part of the animal is called foot. It is entirely formed of test, dirty in colour and rough due to adhered sand particle and other foreign bodies. The foot helps in anchoring the body proper to the sandy bottom. A fully extended foot may attain a length of 3.0 to 4.0 cm. Besides attachment and anchorage, the foot acts as a balancer to keep the body erect when detached from the substratum.
Orientation of the body: The body of the herdmania has a definite and peculiar orientation. The two flat surfaces represent the lateral left and right sides. The anterior and dorsal ends are demarcated by the branchial and atrial aperture, respectively. Their opposite sides correspondingly represent the posterior and ventral ends which are partly free and partly attached. This unusual orientation is caused by the rotatory changes undergone by the larva during metamorphosis.
Test or tunic: Test or tunic forms a protective jacket around the body and is secreted by the epidermis of mantle. It is also renewed periodically and is wrinkled in appearance with a number of folds and depressions running all over the surface. It is soft, 4.0-8.0 mm thick, leathery and translucent. It acts as an accessory respiratory organ and as a receptor organ.
It cuts like a soft cartilage and is composed of- (1) A clear matrix (2) Corpuscles (3) Interlacing fibrils (4) Branching blood vessels and (5) Calcareous spicules (1) Matrix: The gelatinous matrix or ground substance is made up of a polysaccharide called tunicine. (2) Corpuscles: The cells of matrix are mesodermal in origin and of 6 to 7 different types (a) large eosinophilous cells (b) small amoeboid cells (c) small eosinophilus cells (d) spherical vacuolated cells (e) granular receptor cells (f) small branched nerve cells (g) squamous epithelial cells. (3) Interlacing fibrils: These form a fine network in the test. Some of them are like smooth muscle fibers while others resemble nerve fibers. (4) Blood vessels: These form an anastomosing network in the test. Numerous branches near the surface form oval or pear-shaped terminal knobs or ampullae responsible for red patches visible on the surface of test.
The ampullae serve both as an accessory respiratory and as receptor organs being connected to nerve cells. (5) Spicules: Herdmania has large number of calcareous spicules of two types. All the spicules bear all along their length several equidistant rings of minute spines all pointing in the same direction. They are of two types: (a) Megascleres: The large sized megascleres occur in all the parts of body except the heart.
They are (i) Pipette-shaped megascleres reaching upto 3.5 mm in length. They may be straight or curved with pointed ends, (i) Spindle-shaped megascleres are only 1.5 to 2.5 mm long and more abundant, usually present in bundles.
(b) Microscleres: These are minute, only 40 to 80 long and confined to the test only. They resemble with an alpin in shape. The spicules provide an internal rigid supporting frame work like endoskeleton to keep the mantle firmly fixed to the test, stiffen the walls of blood vessels to avoid their collapse and ward off the predators. Mantle or Body wall : The mantle is suspended inside the test and is attached only at the branchial and atrial apertures forming the corresponding siphons. It is not developed throughout the body.
It is thick, highly muscular on the antero-dorsal side, but thin, transparent on the postero-ventral side. It secretes the test and encloses a large water-filled cavity, the atrium. Histologically it is composed of three cellular layers: (1) Outer epidermis: It is a single layer of flat hexagonal cells. It forms inner lining of both the apertures upto their base. Branchial and Atrial siphon are therefore called stomodaeum and proctodaeum respectively.
This layer of mantle secretes the test. (2) Mesenchyme: It lies beneath the epidermis and is derived from mesoderm. It consists of connective tissue fibers traversed by nerve fibers, muscle fibers, and extensive blood sinuses. Muscle fibers are unstriated arranged in three sets.
Annular muscles surround each siphon in several circular ring and help in their contraction. The more numerous longitudinal muscles start from branchial and atrial apertures radiate beneath the annular muscles upto the middle of the body on each side. They bring about the contraction of the body.
A few branchio-atrial muscles extend deeper between the two siphons. (3) Inner epidermis: It is the inner ectodermal single layer of flat polygonal cells lining the atrial cavity.