Question

Classification of phylum Porifera is based upon
A)Nutrition
B)Spicules
C)Locomotion
D)Reproduction

Answer 1

Hint: Phylum Porifera is also termed as sponges (pore bearing animals). These organisms have the body made up of many minute pores. Almost all sponges show an internal skeleton located in the mesophyll layer.

Complete answer:
Porifera is the phylum that is the constitution of the sponges. They do not possess true tissues, have no body symmetry, and are sessile. The classification of phylum Porifera is usually done upon the presence and composition of spicules.
They are placed into 3 classes:
Class: Calcarea
Class: Hexactinellida
Class: Demospongiae
Class Calcarea: The organisms present in this class show the presence of skeletons that are composed of calcareous spicules.
Examples include Leucosolenia, Sycon.
Class Hexactinellida: The organisms of this class show a skeleton that is composed of siliceous spicules which have six rays. Examples includes Euplectella, Hyalonema
Class Demospongiae: The organisms of this class that show presence of skeletons made up of spongin fibres or of sponging fibres with siliceous spicules or may be absent. Examples include Euspogia, Spongilla.

Skeletal structures of sponges are composed of spicules and spongin fibres. Spicules are seen as needle-like structures formed with carbonates of lime or silica. The spicules are made up of the major part of the skeletal system in sponges which are secreted by special mesenchymal cells known as scleroblasts. They usually occur in the shape of spines or rays that radiate from the centre. According to the size, spicules are divided into two major types: Megascleres and microscleres. Megascleres are large-sized spicules and these constitute the main supporting framework of the sponge body. Microscleres are smaller in size and they are present in the mesenchyme.

Hence, the correct answer is option (B)

Note: The invertebrates are animals that show presence of bony structures such as the cranium and vertebrae. They show the presence of some external or internal structures that protect the soft parts of their body. In invertebrates such as sponges it is composed of calcareous and siliceous spicules.