It is a marine water-dwelling acorn worm (Enteropneusta). Blanoglossus cannot survive in freshwater. It belongs to hemichordates. it is an “evolutionary link” between invertebrates and vertebrates.
- Discovered by J.F. Eschscholtz in 1825 in Mashail Island. He described it as a worm-like holothurian. Kowalewski (1865) discovered the gill – slits in Blanoglossus, led to the creation of a special class Enteropneusta by Gegenbaur (1870).
- Bateson (1885) included them in Phylum Chordata. Hyman (1959), however, placed them near Echinodermata and gave Hemichordata a status of an independent phylum.
Classification of Balanoglossus:
- Kingdom: Animalia
- Phylum: Hemichordata
- Class: Enteropneusta
- Order: Enteropneusta
- family: ptychoderidae
- Genus: Blanoglossus
General characters of Balanoglossus:
It has a notochord in the upper part of the body and no nerve chord. It possesses stomochord, (a gut chord within the collar) Term stomochord given by Willy (1899) and Dawydoff 1948).
Habitat: Balanoglossus is a tuberculous or burrowing and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans
Head: its head is small between 5mm to 2.5mm
The body is divided into proboscis (protosome), collar (mesosome), and trunk (metasome)
- The body cavity is enterocelous
- Post-anal tail is absent.
Digestive system: They possess a complete alimentary canal and digestive system.
Respiration: through gills and body surface.
- The blood vascular system is open, or lacunar type, consisting of colorless blood.
Excretion: by single glomerulus gland, commonly known as proboscis gland.
Development: Both direct and indirect. (Tornaria larva in indirect development.)
- Development: It consists mesoelecithal egg.
- It undergoes holoblastic cleavage. Because of cleavage, it gives Morula, and blastula stages. In the Blastula stage, a cavity is presently called Blastocoel.
- The blastula is round in the beginning then it becomes flat, it undergoes invagination and gives a two-layered gastrula. This stage will show blastopore, which will slowly closer. This stage will develop cilia.
- It will also develop an apical tuft. It shows ectoderm and endoderm. It will develop
enterococci coelom. At this stage, the ciliated embryo will be liberated into water. It elongatesand transforms into tornaria” larva
Burrow style: Balanoglossus clavigerus is f-shaped excavated in a sandy bottom
Affinities with Annelids: Spengel (1893) suggested affinities of Balanoglossus with annelids.
Affinities with Echinodermata:
- The affinities of Balanoglossus with echinoderms were first suggested by Metschinkoff (1865).
- There are no similarities between adults of echinoderms and Balanoglossus except in the presence of the intraepidermal nervous system.
- The similarities between tornaria larva and bipinnaria and other larval forms of echinoderms are as follows:
- They are pelagic and transparent
- The early development follows the same pattern.
- The blastopore becomes the anus.
- The coelom is enterocoelic as in pogonophores
- The coelom when first formed is divided into three anteroposterior parts, the protocoel, mesocoel, and metacoel in Balanoglossus and axocoel, hydrocoel and somatocoel in echinoderms. The hydrocoel opens to the exterior by pore as the protocoel and mesocoel do in Balanoglossus. The proboscis pore of Balanoglossus is compared with the water pore.
- The heart vesicle of Balanoglossus is compared with the madreporic vesicle of echinoderm larvae. These structures are closely related to excretory structures, the glomerulus in Balanoglossus, and the axial gland in echinoderms.
- The alimentary canal is divided into three parts, the esophagus, stomach, and intestine.
- The resemblance in ciliated bands of these larval forms is regarded as a superficial character.
Affinities with Chordata: William Bateson was the first to advocate chordate affinities of Enteropneusta and include them in Phylum Chordata. They were supposed to possess the notochord, the gill slits, and the dorsal tubular nerve cord which are three unique characteristics of the chordates.
Important objective questions related to Balanoglossus:
1. Balanoglossus belongs to the group:
2. Balanoglossus is commonly known as:
(a) Snake worm
(b) Acorn worm
3.Larval form of Hemichordata
4. Advanced chordates originated from:
5.Musculature in Balanoglossus is:
6.Coelom in Balanoglossus is:
7. Balanoglossus is:
(a) Surface feeder
(b) Bottom feeder
(c) Column feeder
(d) Ciliary feeder
8. Which of the following is true for the blood vascular system of Balanoglossus?
(b) Absence of central sinus
(c) Blood is colorless
(d) Arteries and veins are absent
9. Body in hemichordate is:
(b) Radially symmetrical
10. Fertilization in Hemichordata is:
(b) External in freshwater
(c) External in seawater
(d) External in air
Answers: 1(d) 2 (b) 3 (a) 4(c) 5 (a) 6(b) 7 (d) 8 (c) 9(d) 10 (c)
You can also read:
- Phylum Porifera
- Phylum Coelenterata
- Phylum Ctenophora
- Phylum Platyhelminthes
Thank you 🙂