Development of Balanoglossus
The development of Balanoglossus is indirect in which a pelagic or pelagic larval stage is found called tornaria.
Fertilization
During the breeding season (May to June), the mature sperm and egg are released from the water of the environment where fertilization takes place. Thus, external fertilization takes place.
EMBRYONIC OR PRELARVAL DEVELOPMENT
The pre-larval development of Balanoglossus is similar to that of a bronchiostoma. A zygote or zygote is formed as a result of fertilization. Its cleavage is holoblastic, almost equal and radial type. As a result, a lobule, tutak or morula is formed of colobules or blastomeres. Due to the reorganization of its coeloblasts, it acquires the shape of a single-layered, hollow and spherical embryo, which is called a corolla, blastula or coeloblastula. Its fluid-filled central cavity is called a corpus callosum or blastocoel. Blastula formation is completed 6 to 15 hours after fertilization. Within 12 to 24 hours an invagination begins to form in the blastula. It gradually deepens to form an archenteron, which opens out through a corm or blastopore. The blastopore soon closes. Now the embryo is called the tubercle or gastrula. It increases in length along the fore-posterior axis. The anterior end of its mandible forms a coelomic vesicle by a narrowing which is called protocoel. Thus, the origin of coelom is enterocoelic. The remaining part of the intestine forms the future alimentary canal. The shape of the protocele becomes triangular. One end of it is attached to the inner plane of the apical thickening. Its other end opens out through a hole, the stomata or hydropore, on the surface of the embryo. The protocele and hydropore of the embryo, respectively, of the adult.
Proboscis make up the coelom and proboscis pore. The coelom of the collar and torso originates independent of the development of the protocele and forms solid involutions of the hindgut.
LARVAL DEVELOPMENT
Simultaneously with the formation of the protocele, the inner end of the initial alimentary canal turns towards the substratum and opens out through the mouth. The esophagus, stomach and bowel are now formed in the alimentary canal. The bowel opens outside in place of the blastopore closed by the anus. By this time, the embryo is evenly covered with cilia. It breaks free from the eggshell and becomes a free-swimming larva called tornaria.
TORNARIA LARVA
1. Structure
First J. Muller discovered the Tornaria larva in 1850. He guessed that it was probably the lacquer of a starfish. Later studies showed it to be the lacquer of Balanoglossus clavigerus. Due to its rotation in circles, it has been named ‘Torneria’. It is transparent and shiny. Its oval body size is up to 3 mm. The ventral mouth near its equatorial plane, the anus at the posterior end, and the alimentary canal are differentiated into esophagus, stomach, and bowel. On the outer plane of the body, there are two plates of the cilium.
The circumoral ciliary band terminates in a postoral loop forming a long curved or coiled path on the anterior surface. Its cilia are small and helpful in collecting food. The posterior ciliary band or telotroch forms a ring in front of the anus. Its cilia are elongated and act like movable organs. At the anterior end of the lacquer there is an apical plate composed of thick epidermal cells. On this 2 eyespots or ocelli and a bunch of sensory ciliates are found which is called apical tuft or ciliary organ. The protocoel or proboscis coelom is like a thin walled sac which opens outwards by a hydropore or proboscis pore. There is a pulsating heart vesicle on the right side of the hydropore. The coelom of the collar and torso appears in the more developed lacquer.
2. Metamorphosis
The laca leads a freely floating planktonic life and feeds on micro-organisms.
It becomes an adult worm by metamorphosis. During metamorphosis, the body becomes smaller in size, perhaps due to the loss of water. Transparency, ciliary septum, sensory cilia and eye dots are lost. As a result of two narrowings, the body gets divided into trunk, collar and trunk. The trunk, region increases in length. The buccal diverticulum and gill-slits appear as outgrowths of the alimentary canal from the hydropore. The worm now starts living the benthonic life of an adult by sitting at the bottom of the water. Direct development takes place in Saccoglossus. It does not form torneria lacquer.