BSc 1st Year Lower Non-chordates Sycon Sample Model Practice Question Answer Papers

Q.6. Give an account of reproduction and development in Sycon.

Or

Describe the life history of Sycon.

Ans.6. 1. Asexual Reproduction

Asexual reproduction occurs by:

1. Budding

2. Regeneration

1. Budding: During favourable conditions Syconreproduces by budding. The buds arise basally near its attachment and then constrict off after sometime to lead independent existence.

Sometimes these remain attached and grow increasing sponge body.

2. Regeneration : Sponges possess great power of regeneration. These not only replace the parts lost during injury, but any piece of the body is capable of growing into a complete sponge. The process is very slow and is completed in months or years. Gemmules are not formed in Sycon.

2. Sexual Reproduction

Both male and female gametes are formed inside the body of the same animal (i.e. hermaphrodite) but definite sex organs are lacking in sponges. Gametes develop from the specialised amoeboid cells, called the archaeocytes found in mesenchyme.

Spermatogenesis: The sperms are produced from an enlarged amoebocyte which is known as sperm mother cell or spermatogonium and is found in the mesenchyme immediately below the choanocytes. It gets surrounded by one or more flattened cover cells either derived by division of spermatogonium or from other amoebocytes. The spermatocyte divides two to three times and cells develop into sperms. The mature sperm consists of a rounded head with a nucleus and a long vibratile tail.

Oogenesis: The egg mother cell or ovocyte is differentiated into an enlarged amoebocyte with a large nucleus and conspicuous nucleolus.

It increases in size and stores food material, being nourished by the specialised choanocytes known as trophocytes or nurse cells. Its nucleus undergoes two maturation divisions (meiosis) to form ovum which projects into the radial canal.

Fertilisation: The fertilisation is internal and cross-fertilisation occurs as a rule.

The sperm immediately enters the nurse cell or an adjacent choanocyte, which becomes amoeboid and fuses with the ovum liberating the sperm. The nuclei of ovum and sperm fuse to form zygote.

Development

1. Cleavage: The divisions are holoblastic and development occurs inside the body of sponge. First three divisions are vertical producing a pyramidal plate of 8 cells. The fourth division is horizontal and divides the blastomeres unequally into 8 micromeres and 8 macromeres. A cavity develops in between them and the embryo enters the blastula phase. The micromeres divide more rapidly and develop flagella at their free ends, while the macromeres become rounded and granular. This represents stomoblastula.

3. Amphiblastula: The stomoblastula undergoes inversion and the flagellated ends of flagellated cells come to lie to the exterior. The flagellated stomoblastula larva is known as amphiblastula larva. The fully formed  amphiblastula is set free into the radial canal. It escapes through the osculum along with the water current and leads a free swimming existence. It undergoes gastrulation by invagination and emboly. The flagellated cells invaginate into the blastocoel and the macromeres grow over them by rapid multiplication. Thus a gastrula is formed.

5. Metamorphosis or Post-larval period: The gastrula adheres to the substratum by its blastoporal end and undergoes metamorphosis to form the adult Sycon. The larva lengthens into a cylinder and develops osculum at the free distal end. The non-granular flagellated cells of endoderm form choanocytes. The granular non-flagellated cells of ectoderm give rise to the pinacocytes of the dermal epithelium. The mesenchyme and its cells are derived both the layers. The body wall gets perforated by ostia and the olynthus stage is attained by Sycon which later on assumes the syconoid structure.

BSc 1st Year Lower Non-chordates Sycon Sample Model Practice Question Answer Papers