Question

In Echinoderms, ossicles are-
A) Exoskeletal plates
B) Endoskeletal plates
C) Regions of ambulacral grooves
D) Regions from where oral tentacles develop

Answer 1

Hint: Echinoderms are exclusively marine animals. They are triploblastic and they contain ossicles which are small calcareous elements. They are found in marine echinoderms in different forms and depict unique arrangements like in Starfishes, Sea cucumbers, etc. They form a part of the water vascular system also.

Complete answer:
In echinoderms, ossicles are endoskeletal plates. The correct option is (B).
Some key points regarding ossicles in phylum Echinodermata are as follows-
• These are embedded in the wall of echinoderms in the form of endoskeletal plates.
• The main purpose is to provide rigidity and protection.
• These are formed intracellularly by specialized secretory cells known as sclerocytes.
• Each ossicle is formed of microcrystals of calcite structured in a 3-D lattice known as a stereom.
• The space between the microcrystals is known as the stroma.
• These are embedded in a tough connective tissue which is a part of the endoskeleton.
• All the ossicles are covered by a thin layer of the epidermis.
Different types of ossicles-
• Plates- They form the skeletal covering for sea stars and Sea Urchins.
• Spines- These projects from the body. They are protective and help in locomotion also.
• Pedicellariae- These are compound ossicles. These are present in Star Fishes.
• Paxillae- These are pillar-shaped ossicles with flat tops. These are the covering of Sea stars- Luidia.

Note:
Echinoderms like all other aquatic organisms possess specialized ambulacral parts (water vascular system)and it consists of the following as mentioned below-
• Ossicles,
• Plates,
• Spines and
• suckers in the ambulacral groove.
The above question demands the origin/structure of ossicles and not the location of ossicles.