Arms, pedicellariae, and spines are absent in which of the following echinoderms?
A. Starfish
B. Sea cucumber
C. Sea lily
D. Sea urchin

Hint: Echinoderms come under the kingdom Animalia. They are the major group of only marine animals. The echinoderms are characterized by the hard, spiny covering or skin.

Complete Answer:
Arms generally refer to limbs. Spines are hard, needle like structures that help in defense of any organism. Pedicellariae are also the defensive organs like a minute pincer.

1. Star fishes: Starfish is also known as the sea start. It is a star shaped echinoderm belonging to class Asteroidea that’s why they are also called as the Asteroids.
- Starfishes are the marine invertebrates. Starfishes usually have 5 arms and their upper surface is spiny that means they contain spines.
- In sea fishes, there are no pedicellariae.

2. Sea cucumber: Sea cucumbers belong to the class Holothuroidea. They are marine animals with leathery skin and elongated bodies.
- Sea cucumbers are found on the seafloor world wide. Sea cucumber helps in the breakdown of detritus and other organic matter, and helps in recycling nutrients in the marine ecosystem. Sea cucumbers have no arms, no spines and no pedicellariae.

3. Sea Lily: Sea lilies belong to the class Crinoidea of phylum Echinodermata.
- Sea lilies resemble the plant more than an animal. The members of class crinoidea include the feather stars.
- Sea lilies have branched arms. They have no spines and pedicellariae.

4. Sea urchin: Sea urchin belong to the class Echinoidea of phylum Echinodermata.
They are typically spiny.
- Sea urchins have spines and pedicellariae. They have no arms and no spines.

Hence, option b is the right answer for this question.

Note: Echinoderms have radial symmetry. The members of echinoderms have five or multiple of five arms. They all have a shell which is made up of calcium carbonate and covers the skin.