Question

8 briefly explain how radial symmetry and the lack of cephalization usually associated with it can have adaptive value for sessile animals and be favored by natural selection.
9 fill in table 15 - 5 with the appropriate characteristics for each class of echinoderms covered to compare the adaptations to different lifestyles among members of these groups.
table 15 - 5 comparison of major characteristics among echinoderm classes

characteristic	crinoidea	ophiuroidea	echinoidea	asteroidea	holothuroidea
tube feet (present or absent; lack or possess suckers)
ossicles (well - developed, reduced, etc.)
feeding method
location of madreporite
spine structure

Explanation:

Question 8:

Sessile animals are fixed - in - place. Radial symmetry allows them to interact with the environment from all directions equally. Since they don't move to seek food or mates, there's no need for a head (cephalization). This symmetry helps in capturing food that comes from any direction and in responding to threats from all around.

Question 9:

• Crinoidia: Shape of arms: Feathery and branched; Tube feet: Present, with suckers; Ossicles: Well - developed; Feeding method: Filter - feeding; Location of madreporite: On the oral surface; Spine structure: Small and often covered by skin.
• Ophiuroidea: Shape of arms: Long, slender, and flexible; Tube feet: Present, no suckers; Ossicles: Well - developed; Feeding method: Deposit - feeding or scavenging; Location of madreporite: On the aboral surface; Spine structure: Modified into spines and pedicellariae.
• Echinoidea: Shape of arms: Absent; Tube feet: Present, with suckers; Ossicles: Well - developed, form a test; Feeding method: Grazing or scavenging; Location of madreporite: On the aboral surface; Spine structure: Prominent, movable.
• Asteroidea: Shape of arms: Broad and flexible; Tube feet: Present, with suckers; Ossicles: Well - developed; Feeding method: Predation (everting stomach); Location of madreporite: On the aboral surface; Spine structure: Variable, often short.
• Holothuroidea: Shape of arms: Absent; Tube feet: Present, some with suckers; Ossicles: Reduced; Feeding method: Deposit - feeding; Location of madreporite: On the oral surface; Spine structure: Reduced or absent.

Answer:

Question 8: Radial symmetry helps sessile animals interact with the environment from all directions, and lack of cephalization is not a hindrance as they don't move to seek specific stimuli.

Question 9:

Characteristic	Crinoidea	Ophiuroidea	Echinoidea	Asteroidea	Holothuroidea
Tube feet (present or absent; lack or possess suckers)	Present, with suckers	Present, no suckers	Present, with suckers	Present, with suckers	Present, some with suckers
Ossicles (well - developed, reduced, etc.)	Well - developed	Well - developed	Well - developed, form a test	Well - developed	Reduced
Feeding method	Filter - feeding	Deposit - feeding or scavenging	Grazing or scavenging	Predation (everting stomach)	Deposit - feeding
Location of madreporite	On the oral surface	On the aboral surface	On the aboral surface	On the aboral surface	On the oral surface
Spine structure	Small and often covered by skin	Modified into spines and pedicellariae	Prominent, movable	Variable, often short	Reduced or absent