ELPHIDIUM - STRUCTURE AND DIFFERENCES BETWEEN MICROSPHERIC & MEGALOSPHERIC FORMS

Phylum : Protozoa

Class : Rhizopoda

Order : Foraminifera

Genus: Elphidium

Species: crispum

      STRUCTURE OF ELPHIDIUM (POLYSTOMELLA)

Elphidium is also called 'Polystomella is a 'dimorphic rhizopod'. It is found creeping on Sea weeds to a depth of 300fathoms. It is a unicellular microscopic protozoan, and" 1 mm in diameter. It is pale yellow in colour. It lives in marine water. Body of Elphidium is covered with a hard and translucent shell made up of calcium carbonate and small amounts of silica and magnesium sulphate. The shell is biconvex, polythalamus or multilocular (many chambered) and perforated. perforate calcareous shell consisting of 45 to 50 chambers. The surface of the shell is chiseled. The chambers of the shell are V-shaped, laid down serially and arranged in a flat spiral in which each whorl of chambers overlaps the previous whorl. The overlapping portions are known as alar processes.

Due to the overlapping of the chambers only, the last chamber is visible from outside. The hinder margin of each chamber has a row of numerous minute backwardly directed, hollow, blind protoplasmic pockets called retral processes. The adjacent chambers remain separated from each other by perforated septa.

The chambers are interconnected or communicate with each other as well as with the exterior through minute pores present in the septa. The outer whorl opens to the outside by a row of large pores. The chambers of the shell originate from the initial chamber known as proloculum which may be small or large in size.

The small proloculum is known as microsphere and the shell having small proloculum shall be called microspheric, whereas the large proloculum is known as megalosphere and the shell having large proloculum is called megalospheric.

Cytoplasm:

The cytoplasm fills all the chambers of the shell, called inner cytoplasm. Besides, a thin layer of cytoplasm covers the shell from outside, called outer cytoplasm. The cytoplasm is not differentiated into ectoplasm and endoplasm. However, the inner cytoplasm contains nucleus or nuclei, food particles, minute vacuoles, Golgi apparatuses, mitochondria, endoplasmic reticulum, ribosomes and brown granules or xanthosomes which are apparently waste matter. Contractile vacuoles are not found.

The chambers are filled with the cytoplasm.

1. The cytoplasm contains one or many nuclei.

2. Contractile vacuole is absent.

3. Mouth is absent.

4. Cytoplasm contains food vacuoles. They take up the process of digestion.

                 Elphidium structure                          Chambers                               Reticulopodia

Nucleus:

The cytoplasm of megalospheric individuals contains single nucleus, while those of microsphoric individuals contains many nuclei. The nucleus is of vesicular nature and possesses many nucleoli in its nucleoplasm.

Rhizopodia:

The pseudopodia of Elphidium are branched and anastomosing of numerous fine and long slender thread-like structures. This type of pseudopodia are characteristically called reticulopodia, rhizopodia or myxopodia. Each rhizopodium is made of an inner fibrillar axis and the outer fluid-like cortex.

The streaming circulation of cytoplasm has been locomotory in function and feeding nets for catching diatoms on which animal feeds.

Dimorphism: Polystomella exhibits dimorphism. The individual occurs in two distinct forms.

1. Megalosperic form.

2. Microspheric form.

1. Megalospheric form:

Its proloculum is big in size. A single large nucleus is present in one of the chambers. It takes up sexual reproduction.

2. Microspheric form: Its proloculum is small in size. Many nuclei are present in the cytoplasm. This form reproduces by asexual reproduction.

Differences between Microspheric & Megalospheric forms

Macoscopic form 

Microscopic form 

a) The first formed chamber or Proloculus is large

The first formed chamber or Proloculus is small

b) Initial chamber is naked like other chambers

Initial chamber is encloused in a thick capsule like cyst

c) Nucleus one, large and laterally placed

Nuclei many, small and scattered throughout the cytoplasmic mass

d) Available in large scale

Available in small scale

e) Chromidia absent

Chromidia present

f) Produced Asexually

Produced Sexually

References:

E. L. Jordan and PS Varma: Invertibrate Zoology

P. S Varma: Invertibrate Zoology, 2001

R.L.Kotpal: Modern text book of zoology: invertebrates 12th Edition   

Barnes, R. D. ( 1968). Invertebrate Zoology, 2nd ed., Saunders, Philadelphia.

                                                                                                                   Dr. Karri. Rama Rao

                                                                                                                Govt. Degree College

                                                                                                         TEKKALI, Srikakulam Dt.