Study of Spicules and Gemmules in sponges

Permanent Slides-Spicules and Gemmules in sponges

⚫Spicules in sponges 

    1] The Spicules constitute major part of skeletal system, which are secreted by special mesenchymal cells called scleroblasts. The are in the shape of spines or rays that radiate from the centre.

    2] Spicules possess a core of organic material around which is deposited either calcium carbonate or silica. Spicules are of two types :- calcareous, made of calcium carbonate and siliceous made of silica.

    3] According to the size, Spicules are classified into two major types :-

        i] Megascleres :- Large-sized & constitute main supporting framework of sponge body.

        ii] Microscalers :- Smaller in size and occur in the mesenchyme.

    4] Based on the number of axis present in the rays spicules may be of three types :-

            (i)Monoaxon (ii)Triaxon (iii)Polyaxon

        (i) Monoaxon :- These Spicules grow along a single axis. These may be straight needle-like as rod-like or may be curved. Their ends nay be pointed, knobbed or hooked. If Spicules has only one pointed end it is called Monoactinal.

        (ii) Triaxon :- Triaxon Spicules has three axes that cross one another at right angles to produce six-rays. These are found only in class Hexactinellida.

        Tetraxon :- Triradiate are also tetraxon in which one of the four rays is lost. In calthrops all rays are equal.

        (iii) Polyaxon :- In this type of Spicules, several equal rays radiate from a central point. They are common among the glass sponges.

fig. Spicules and spongin (A-Monoactinal monaxin; B-Diactinal monaxon;

C-Curved monaxon; D-Monaxon with hooked end; E-Tetraxon; F-Triradiate;

G-Calthrops; H-Hexactiral triaxon; I&J-Polyaxon; K-spongin fibres.

⚫Gemmule

    1] It is the asexual reproductive body.

    2] These bodies develop only in unfavorable conditions.

    3] It is actually a round ball-like structure comprising of central mass of archeocyte cells & a thick peripheral layer of amphidiscs.

    4] Actually speaking the peripheral layer is made up of an outer membrane, a thick pneumatic layer and an membrane.

    5] The pneumatic layer is full of air chambers and help in floating.

    6] The central mass communicates to outside by micropyle.

    7] In addition to archeocytes are also present a few trophocytes and scleroblast cells.

    8] The trophocytes are full of food in the form of glyco & lipoproteins.

    9] The archeocytes are reproductive cells. On the return of favorable conditions the Gemmules germinate and gives rise to young sponges.

        

            fig. Gemmule