Question

Compare and contrast between terrestrial and marine diversity within same phylum

Arthropoda: Cheliform ----> Marine vs Terrestrial

Arthropoda: Crustacea ----> Marine vs Terrestrial

Answer 1

Cheliceriformes

Cheliceriformes (ke-LIS-er-i FORM-es) is derived from three roots than mean "clawed horn forms" [clawed -cheli Gr. (χηλή); horn -ceros (κέρας); formes (Latin for forms)]. This is a reference to the clawed feeding structures (chelicerae) that are common to this subphylum. The name was coined by Schram and Hedgepeth (1978) to recognize a higher taxon with Chelicerata and Pycnogonida as sister groups.

The Pycnogonids

The Sea Spiders are entirely marine. The opisthosoma (abdomen) is very small and unsegmented. The prosoma (cephalothorax), however, is divided into a head with a cylindrical proboscis and three pairs of appendages (chelicerae, pedipalps, ovigerous legs - non-ambulatory legs used for carrying eggs). The trunk is has four to six segments, each with one pair of legs borne on the end of large lateral trunk processes. The legs often are very long and can have a span of 75 cm. They have two pairs of eyes on a rounded tubercle that projects from the posterior head region. The gut has lateral caecae that extend into parts of the legs, as do parts of the gonads. Despite the size of the Sea Spiders, they have no excretory or respiratory organs. The haemocoel divided into upper and lower sections by a horizontal membrane, and the nervous system is diffuse, not concentrated. The male broods the eggs, which hatch as a protonymphal larval stage with three pairs of appendages. The Sea Spiders were given as sisters to the other living arthropods in the analysis of Waggoner (1996).

The Merostomates

The Horshoe Crabs are marine, crab-like animals with a heavy exoskeleton. The prosoma is a large horseshoe-shaped carapace, which is separated from the opisthosoma by a hinge. The telson, which forms the tail spine, is long and substantial. Like most chelicerates, the prosoma has two lateral compound eyes and 2 median ocelli.   The chelicerae are small, and the pedipalps are leg-like and end in claws. Likewise, the walking legs have claws, except the last pair, which have leaf-like processes used for burrowing. The spiny gnathobases of the limbs macerate food. The first pair of opisthosomal appendages form a covering over the reproductive openings. The second through the sixth pairs of opisthosomal appendages form swimming and gas exchange organs (called book gills). The opisthosoma is relatively small, unsegmented, and has lateral spines. Excretory organs are associated with the coxae, the most proximal segments of the legs. Fertilization is external, producing a benthic "trilobite" larva. Maturation takes up to ten years.

Eurypterids were among the top predators of the middle and late Paleozoic Seas. Sometimes called Sea Scorpions, eurypterids grew to be more than 2 meters long, though most were smaller. They likely walked over the bottom in search of prey. The placement of legs in some taxa even suggest that they could walk out onto land.

The Arachnids

These are the spiders, ticks, mites, scorpions, whip scorpions , and ambylopygids, which are all essentially terrestrial animals. The exoskeleton is variable, but generally light. The prosoma is entirely (or partly) covered by a carapace, and they usually have four pairs of walking legs. The opisthosoma has thirteen segments plus a telson, which is often reduced. Furthermore, the abdominal appendages are highly modified or, in many cases, absent. The respiratory organs are book lungs or tracheae. The animals have many eyes, but they usually are simple and without compound eyes.  

The arachnids consume liquid (usually pre-liquified animal) food using a pumping pharynx. They inject prey with venom through the chelicerae, which begins the process of digestion. Tarantulas can eat prey as large as small rodents and birds, which they reduce to a mass of bones and fur/feathers in less than two days. The toxins can be quite potent and deadly to humans.  

Young arachnids look like the adults. That is, they do not go through a larval stage. However, some of the young can have three pairs of walking legs and add the fourth pair during successive molts.

Crustacean

Crustaceans (Crustacea /krʌˈsteɪʃə/) form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, woodlice, and barnacles. crustaceans are found living in both aquatic and terrestrial environments, most are adapted to life in the water . Regardless of where they are found, crustaceans fill many roles within the ecosystem, providing vital support to the food web . Some are amphibious, meaning they are able to leave the water, often to scavenge for food on land. Certain crabs spend so much time on land that they are considered land crabs, although they still need to return to water when it is time for their offspring to hatch. Aquatic crustaceans can live in marine or freshwater habitats, in addition to inland brines containing very high salt concentrations . Crustaceans have adapted to many different habitats within the water; ranging from open waters, to the sandy sea bottom, to ocean trenches, to buried in the sea bottom itself, to mud, to rocky outcrops .