Question

check your progress
3.1 what structure in the cardiac chamber of the stomach allows crayfish to digest their food mechanically?
3.2 what organ secretes enzymes into the pyloric chamber of the stomach to assist in the chemical digestion of food?
3.3 what type of circulatory system does the crayfish possess, open or closed? what anatomical evidence is present to support your answer?
3.4 where on the body surface of the crayfish are the external openings for the excretory organs (green glands) located?
3.5 how do the relative positions of the brain and the nerve cord in crayfish compare to the positions of the brain and the nerve cord in the earthworm? is this evidence of common ancestry?

exercise 14 - 7
insects (subphylum hexapoda, class insecta)
materials needed
□ preserved grasshopper (romalea)
□ dissection tools
□ dissecting pan
□ dissecting pins
□ latex gloves
□ dissecting microscope
with nearly 1 million described species, the class insecta (part of the subphylum hexapoda) is by far the largest, most diverse, and most widespread class of arthropods. more than half of all named animal species are insects. the united states alone has more than 100,000 different named species of insects. though their diversity is astonishing, the members of this group share several basic similarities (fig. 14.19). all insects have three pairs of uniramous (unbranched) walking appendages, a single pair of antennae, and three distinct body regions: the head, thorax, and abdomen. insects are primarily terrestrial creatures, and their evolution has been shaped by advances that have allowed them to thrive despite the unique set of challenges of a terrestrial environment, including:

1. stronger support systems and stronger walking appendages to overcome the forces of gravity,
2. waxy cuticles built to withstand the osmotic stresses of “dry” air, yet adapted for gas exchange,
3. the ability to fertilize eggs internally to prevent desiccation of exposed gametes,
4. specialized respiratory, excretory, and digestive structures modified to conserve water, and
5. appendages modified into wings to take advantage of aerial habitats.

in the following exercise the grasshopper will serve as our representative example of an insect that displays all of these adaptations to terrestrial life.
chapter 14 arthropods 269

Explanation:

3.1 The gastric mill in the cardiac chamber of the crayfish stomach grinds food mechanically.
3.2 The hepatopancreas secretes enzymes into the pyloric chamber for chemical digestion.
3.3 Crayfish have an open circulatory system. Blood is pumped into sinuses and bathes organs directly.
3.4 The external openings of the green glands (excretory organs) in crayfish are located at the base of the antennae.
3.5 In crayfish, the brain is dorsal and the nerve - cord is ventral. In earthworms, the brain is dorsal and the nerve - cord is ventral too. This similarity is not strong evidence of common ancestry as similar body plans can evolve independently.

Answer:

3.1 The gastric mill allows crayfish to digest food mechanically.
3.2 The hepatopancreas secretes enzymes into the pyloric chamber.
3.3 Crayfish have an open circulatory system. Evidence: Blood is pumped into sinuses and bathes organs directly.
3.4 The external openings of the green glands are at the base of the antennae.
3.5 In crayfish, the brain is dorsal and the nerve - cord is ventral. In earthworms, the brain is dorsal and the nerve - cord is ventral. This is not strong evidence of common ancestry.