Arthropoda

We've described the ancestral tagma of the two of the arthropod subphyla; how are the tagma and appendages arranged in a Chelicerata? (p. 82)

Chelicerates have two tagma: an anterior prosoma and a posterior opisthosoma, also referred to as the cephalothorax and abdomen, respectively. In some chelicerates, scorpions for example, the opisthosoma is further divided into a mesosoma and metasoma. Other chelicerates, horseshoe crabs, have a postanal, unsegmented extension of the most posterior part of the body, a telson. The prosoma has six pairs of appendages with pedipalps, chelicera, and four pairs of walking legs. There are no eyes or antennae on the tagma and as such no real head. This is the reason some biologists prefer the term prosoma to cephalothorax. There are no appendages on the opisthosoma, although the protective covers of the book gills in horseshoe crabs and the spinnerettes in spiders may be homologues of ancestral appendages found on the tagma.

Why don't crustaceans have a waterproof epicuticle? What's the consequence of not having one? (p. 83)

Although we can't say for certain, waterproofing probably wasn't a problem for marine crustaceans, and that's probably the best explanation for why a waterproof epicuticle is missing--it wasn't needed. It may also explain, with the few exceptions that live in moist terrestrial environments, why crustaceans are only found in the oceans. It was the arthropods that did have a waterproof epicuticle who were able to conquer the terrestrial environment.

Why do the chitinases and proteinases breakdown only the old cuticle and not the new one? (p. 83)

After secreting inactive chitinases and proteinases, the epidermis secretes the first part of the new cuticle, the inner cuticulin formed from highly cross-linked proteins. The hydrolytic enzymes are outside of the cuticulin layer and next to the old cuticle, and once the cuticulin is in place, the enzymes are activated and begin to work. Enzymes are trapped outside the cuticulin because they are too large to get through the fine molecular sieve created by the cuticulin layer and can't reach the new procuticle forming underneath. The proteinases can't digest proteins in the cuticulin because the cross linking makes them resistant to digestion. The only chitin and protein that they can hydrolyze is in the old cuticle, and that's what is digested. The products of digestion, amino acids and glucosamine units, are small enough to cross the cuticulin layer and are used by the epidermis to build new procuticle.

Marine crustaceans and terrestrial arthropods have the same tubular skeleton and appendages. Why are the crustacean's appendages so much larger? (p. 84)

Unlike terrestrial animals marine crustaceans take advantage of the buoyancy of the marine environment to support their larger skeletons. Essentially the water is doing most of the work of supporting the skeleton countering much of the effect of gravity.

Eyes are important in detecting and capturing prey. How do chelicerates do that without compound eyes? (p. 85)

Although some of the earliest chelicerates may have had eyes, now most lack the compound eyes characteristic of the other Arthropoda. To detect prey, terrestrial chelicerates, spiders and scorpions, for example, detect vibrations in either the substrate or the webs they spin. Their bodies are covered with specialized structures, slit sensilla and tricobothria, that detect even the slightest movement of the air around them or in the web they have spun. This poses a problem during mating when the female must identify a potential male as a mate, rather than a meal. As a result, males often use complex courting rituals, and male spiders pluck the strings of the web to identify themselves as mates.

How do crabs and crayfish eliminate metabolic wastes? (p. 85)

In marine species, metabolic wastes are removed primarily by diffusion across the gill surface; the antennal glands, or green glands as they are also called, are involved more in osmoregulation than excretion. The situation is different in freshwater crayfish that live in a hyposoomotic environment--water is constantly diffusing into the animal's tissues. The antennal glands are important in removing this excess water and retaining essential salts. In both marine and freshwater species, salts are pumped into the saccule end of the gland, and then an ultrafiltrate of the hemolymph crosses the membranes carrying with it small molecular weight compounds that include essential salts and ions. The tubule portion of the gland recovers the salts and eliminates water or any nitrogenous waste that may be contained in the filtrate.

How does locomotion differ in the crab compared to crayfish or lobsters? (p. 86)

The main difference in their locomotion is the direction that the animals move. When crayfish "walk," they move forward; crabs move sideways. For the crab to be able to do that, the abdomen is reduced and tucked up and underneath the cephalothorax. This is different from the crayfish or lobster where the muscular abdomen acts as a counterweight to the chelipeds extended in front of the animal as it walks.