Final answer:
Sea urchins have the longest spines at the top of their dome-shaped bodies to help with protection and movement, while sand dollars, adapted to a buried lifestyle, have much shorter spines.
Step-by-step explanation:
In the context of sea urchins and sand dollars, which are marine invertebrates belonging to the class Echinoidea, there is a distinct difference in the placement of their spines. Sea urchins, also referred to as regular echinoids, live on the ocean floor and feature long spines that are part of their endoskeleton, made of calcareous ossicles covered by the epidermis. These spines serve important functions including movement, protection, and interaction with the environment. Conversely, sand dollars are considered irregular echinoids and have a much flatter shape, leading to a different spine structure and length distribution geared towards their life within the sediment on the ocean floor.
The spines of sea urchins are generally longest at the top of the dome-shaped body, away from the substrate, which assists in protection and locomotion over the ocean floor. Meanwhile, sand dollars tend to have much shorter spines relative to sea urchins, as their flattened bodies are adapted to live partly buried in the sediment.