Balanoglossus, Ascidia and Branchiostoma | Phylum Chordata

Let us learn about the comparison between balanoglossus, ascidia and branchiostoma.

Comparison # Balanoglossus:

1. The pharynx is an elon­gated structure which is internally divided into 2 incomplete upper and lower halves by a longi­tudinal parabranchial ridge or Para pharyngeal ridge. The upper one is called the respiratory por­tion and the lower part is the digestive chamber.

2. The pharynx is contai­ned in the branchiogenital region of the trunk.

3. The mouth leads into a short buccal tube which opens into the pharynx.

4. The pharynx leads directly into the straight intestine.

5.The pharynx is attached with the body by the dorsal mesenteries.

6. Absent.

7. The upper respiratory half is perforated by numerous U-shaped pharyngeal slits which increase in number with age. Each pharyngeal slit leads into a branchial sac which, in turn, communicates to the exterior by a gill-pore. In Balanoglossus misakiensis the first four branchial sacs fuse to form a common bran­chial sac opening by a single branchial pore. The gill-bars contain innumerable long vibratile cilia.

8. The cavity is almost uniformly lined by cilia and mucous cells.

9. No such structure is present.

10. Absent

11. Absent

Comparison # Ascidia:

1. The pharynx is a sac­like chamber of large dimension. The internal wall of the pharynx is complicated. In a rela­ted urochordate, Herd- mania, the internal cavity is longitudinally folded to increase the surface area.

2. Extended from the mouth nearly to the base of the body.

3. The mouth is situated at the centre of the velum and placed in the poste­rior end of the oral fun­nel. The mouth opens into the pharynx.

4. The pharynx is commu­nicated with the stom­ach through a narrow oesophagus.

5. Only the ventral side of the pharynx is attached with the mantle along the middle line.

6. A special cavity called the atrium or peribranchial cavity enclo­ses the pharynx on all the sides except the ventral one. The atrium opens to the exterior through the atriopore situated on one antero­lateral side of the body.

7. The pharynx is pierced on all sides except the line of attachment with the mantle by innumer­able vertical stigmata. The stigmata bear series of papillae containing muscles and cilia.

8. The cilia and glandular elements are restricted into definite tracts.

9. A highly developed endostyle is present in the ventral side of the pharynx. Endostyle acts as a fly-paper. The endostyle has two rows of mucous cells separa­ted by rows of ciliated cells. A group of median cells with extremely elongated cilia is pre­sent at the middle.

10. On the mid-ventral line of the pharyngeal cavi­ty, i.e., just opposite to the endostyle, there lies the dorsal lamina from which hangs a number of curved bodies, called the languets.

11. The dorsal lamina and the endostyle are joined by the peripharyngeal ciliary bands.

Comparison # Branchiostoma:

1. The pharynx is a large cylindrical and highly compressed chamber. Like that of Ascidia, the pharyngeal cavity is very much complicated.

2. Extends nearly to the anterior half of the body.

3. The mouth is situated at the end of the vestibule which is preceded by the oral hood and buccal cirri. A wheel organ is present in the vestibule. The mouth leads into the pharynx. Like that of Ascidia, the mouth is guarded by velum.

4. The pharynx opens into a straight midgut via a short oesophagus.

5. Only the dorsal side of the pharynx is united with the body wall.

6. The atrium envelopes the pharynx on ail the sides except the dorsal. The atriopore is located posteriorly.

7. The lateral wall of the pharynx is perforated by obliquely arranged ver­tical gill-slits. There are two types of gill-slits, the primary and sec­ondary ones. There are about 200 gill-slits which increase in num­ber with age. Each gill-slit opens directly into the atrium. The tongue- bars are provided with long cilia.

8. Like that of Ascidia, definite ciliated tracts are also present in Bran­chiostoma

9. Endostyle consisting of tracts of ciliated cells alternating with mucus- secreting cells is present

10. A median groove, called the epipharyngeal or hyper pharyngeal groo­ve, is present opposite to the endostyle.

11. The epipharyngeal groo­ve and the endostyle are connected anteriorly by peripharyngeal ciliated tracts.