An Example of Phylum Ctenophora: Hormiphora

In this article we will discuss about Hormiphora:- 1. Habit and Habitat of Hormiphora 2. Structure of Hormiphora 3. Histology 4. Gastro-Vascular or Enteric System 5. Nervous System 6. Reproductive System 7. Development.

Contents:

1. Habit and Habitat of Hormiphora
2. Structure of Hormiphora
3. Histology of Hormiphora
4. Gastro-Vascular or Enteric System of Hormiphora
5. Nervous System of Hormiphora
6. Reproductive System of Hormiphora
7. Development of Hormiphora

1. Habit and Habitat of Hormiphora:

The basic plan of the ctenophores can be best illustrated with reference to the gener­alised genus, Hormiphora. Hormiphora plumosa is found in the Mediterranean. Its closely allied genus, Pleurobrachia is found all over the world as a common pelagic form. In the present text the genus Hormiphora is described as a typical representative of the phylum. It is a typical example of Ctenophora which includes variety of forms.

Due to diverse adaptive radiation, the different members of this group exhibit wide structural variations. Despite such diversities, the basic organisa­tion remains the same in all the cases.

All the ctenophores have transparent gelatinous bodies and are commonly known as ‘Comb- jellies’ because of the presence of ciliated comb-like plates or Costae on the body. Each plate or band is made up of partially fused long cilia and called a ctene. These plates are of paramount significance because the phylum owes its name to these structures.

2. Structure of Hormiphora:

Hormiphora has a pear-shaped body meas­uring 5-20 mm in diameter (Fig. 13.2A). They are pelagic and are found in the Mediterranean sea. Mouth is situated at the centre of the oral pole and the opposite pole is occu­pied by a complicated and characteristic sense organ usually designated as the aboral sense organ. This sense organ is a modified statocyst and acts as an organ of equilibrium (Fig. 13.2B).

The sense organ which is en­closed within a transverse dome bears a concavity or depression which is internally lined with tall ciliated ectodermal cells. Four very long S-shaped tufts of cilia (balancers or springs) are inter-radially located and arranged at equal distance within the de­pression.

Two slender ciliated areas, called the polar plates are transversely arranged within the depression. The balancers project upward and meet to support a rounded mass of calcareous spherules (statolith). A pair of ciliated furrows pass to swimming plates from the base of each balancer.

The surface bears eight equidistant me­ridional swimming-plates or costae or comb-plates each of which is composed of trans­verse row of long cilia. A pair of very exten­sible, long and solid tentacles bearing a row of lateral branches are present on the two sides of the broader end.

The bases of these two tentacles are encased by sheath within which they can be retracted. The tentacles are composed of a central core which re­mains covered by ectoderm consisting largely of adhesive cells called colloblasts or lasso cells (Fig. 13.3A).

Each colloblast develops from a single cell with its nucleus modified into straight filament. The colloblast has a dorsal convex surface and contains numerous small papil­lae which can adhere to any object. The inner side is produced into a spirally coiled fila­ment around the straight nuclear filament. These filaments extend up to the muscular axis of the tentacle.

3. Histology of Hormiphora:

Ectodermal cells are cuboidal or colum­nar in shape and are ciliated in certain re­gions. These cells are interspersed with nu­merous gland cells, and sometimes certain pigment granules or branched pigment cells (melanophores) are also present.

The sensory cells are of two types (Fig. 13.3C):

(i) One type bears several stiff bristles and

(ii) The other type is with a single stout bristle.

The so-called mesoglea is designated as the collenchyme. The collenchyma is inter­preted as the ectomesoderm and consists of a gelatinous substratum containing cells, con­nective-tissue fibres and muscle fibres.

All these components are ectodermal in origin, except the muscle fibres which are inde­pendent cells arising by the direct conver­sion of the amoeboid cells. The muscles are elongated smooth fibres disposed longitudi­nally and circularly.

4. Gastro-Vascular or Enteric System of Hormiphora:

The mouth leads into pharynx or stomodaeum which has internal ridges. The stomodaeum opens into a cavity known as the stomach or infundibulum from which arises the following canals of the coelenteron (Fig. 13.2B).

(i) Infundibular canal:

It runs upwards up to the aboral pole where it breaks into four small branches of which two open on the surface by the excretory pores and the other two are blind.

(ii) Per-radial canal:

Two canals, one in each direction, extend along the transverse plane of the body.

Each per-radial canal sends:

(a) One stomodaeal canal along the stomodaeum,

(b) One tentacular canal in the ten­tacle and then divides into, and 

(c) Two inter-radial canals, each of which subdivides into two ad-radial canals.

Each ad-radial canal opens within a meridional canal which runs beneath the comb-plate.

5. Nervous System of Hormiphora:

The nervous system is much diffused and consists of a general sub-epithelial nerve plexus with multipolar ganglionic cells and neurites throughout the surface.

6. Reproductive System of Hormiphora:

Both sexes are present in one individual. Gonads are situated in the wall of the me­ridional canals as continuous or discontinu­ous bands. Originating from the endoderm, the matured garnets are discharged into the canals and finally escape through mouth. Fertilisation is external.

7. Development of Hormiphora:

Developmental history of Hormiphora is not well known but the following events as observed in other forms of Ctenophora (par­ticularly in Callianira) will give the idea of development (Fig. 13.4).

Zygote gives rise to four blastomeres by two meridional cleavages (Fig. 13.4A).

The third cleavage is nearly vertical, re­sulting in a curved plate of eight cells, ar­ranged in two rows (Fig. 13.4B). The eight blastomeres divide twice along the horizon­tal plane and give rise in each time eight small cells (micromeres) and eight larger cells (macromeres) (Fig. 13.4C).

The micromeres are the source of ectoderm and the macromeres give rise to endoderm in due course. Micromeres undergo rapid divi­sion and proliferate as a wreath of smalls cells over the macromeres which ultimately grow down as a one-layered sheet. Invagi­nation starts and gastrulation goes on by combined processes of emboly and epiboly (Fig. 13.4E).

Micromeres cover the embryo to become the ectoderm (Fig. 13.4F), four inter- radial bands of small rapidly dividing cells become noticeable which differentiate into comb-rows. Ectoderm at the oral pole invaginates extensively to form stomodaeum from which gastro-vascular cavity arises by active endodermal outgrowth. Biradial sym­metry persists throughout the development.