Xastre
The xastre (pronounced /ˈzæstɚ/) is a huge, eight-legged creature of Dadauar's oceans, one of the top predators of its environment. Though they are nowhere very numerous, xastres are found throughout the seas on the pelagic reefs and the shallower parts of the ocean floors, though not, of course, at the deepest reaches where the Bathybius lies.
Etymology
The name of the xastre comes from the Old Balian word xàstrē, which in turn is an augmentative form of the word xàst, meaning spider. A derivation is sometimes claimed from the name of the xyston (ancient Greek ξυστοόν), a kind of spear, in reference to the animal's hastiform weapons. This, however, is a folk etymology; there is no connection between those words, and the resemblance is coincidental.
While "xastre" has become the most common name used for the creature, and in most widespread languages on Dadauar it's called by some borrowed version of this word, a few do use other names for it. A now archaic word of Drithidian origin still occasionally seen in old documents is "spinnelfisk" (/ˈspɪnəlfɪsk/ or /ˈʃpɪnəlfɪʃk/). Other now little-used names for xastres include "lonchite" and "sneser". Xastres are sometimes called sea spiders, though this name is ambiguous since it is also used for much smaller chelicerates of the order Pantopoda.
Description
An adult xastre's body is about one to six meters long, depending on the species, and consists of three tagmata, each roughly conical in shape (the apex of the cone pointing in the dorsal direction). From front to back, these sections are called the head, the sternon, and the araea; the last is by far the largest. They have ten thin but strong long, segmented legs, covered with setae, three pairs of legs attached to the sternon and two to the araea. The frontmost pair of legs is highly modified, the tips of the legs thickened and hardened into stabbing appendages called quirites (singular quiris) that the xastre uses to strike at its prey.
A xastre's head bears six eyes, the two largest of which are compound. Before the eyes are two filiform antennae. The mouth is located on the bottom of its head, and is surrounded by several mouthparts: a hard anterior plate called the hypostome, two large, forward-pointing deplectors, and four small but dextrous maxillae. None of these mouthparts is used offensively, but once the xastre's prey is killed or subdued by its quirites it is held in place by the deplectors while the maxillae tear off small pieces and feed them into the xastre's mouth.
Anatomy
The xastre's vital organs are concealed and protected by a hard chitinous exoskeleton that also helps the creature hold its form. Numerous apodemes extend from the exoskeleton inth the animal's interior to act as additional supports as well as anchoring points for the xastre's muscles. The xastre has an open circulatory system, with hemolymph bathing its internal organs, but it does have a long, tubular heart running dorsally from front to back, with branches splitting off to reach the farther parts of its body. The main parts of the xastre's nervous system, on the other hand, are mostly ventral, with a series of nerve cords running back from the brain at the bottom of its head, swelling at intervals into ganglia. Above the nerve cord and below the heart is the xastre's alimentary canal. The xastre has three stomachs, one in each tagma; the exact reasons for this are not well understood. In the head is the orœcus, in the sternon the larger mediœcus, and in the araea the still larger amplœcus.
Like most aquatic arthropods, the xastre uses the gills to breathe. Developed from branches of the legs, the gills are feathery appendages that extract oxygen from the water flowing through them. Although there are two pairs of gills in the sternon, the most and the largest gills are located in the araea, much of the volume of which is given over to spacious branchial chambers. The sternal gills seem to be largely vestigial; they are not infrequently lost to disease or parasites, and xastres without them are not appreciably impaired. Because the araeal gills are much more important, they are also much better maintained; specialized appendages within the branchial chambers called særopods keep the gills groomed and cleaned.
Taxonomy and evolution
Though xastres have been compared to both crabs and spiders, they are not closely related to either, though like crabs and spiders they are arthropods. They are conventionally placed in the family Tritholidae within the order Aglaspidida; the exact relation of this order to the more familiar crustaceans and chelicerates remains uncertain. They are, however, relatively closely related to the extinct (on Earth) trilobites.
Although xastres share with insects the division of the body into three tagmata, the origins of these tagmata are distinct. In an insect, the head is made from the fusion of six ancestral somites; the middle section, or thorax, of three, and the caudal tagma, the abdomen, of eleven. The forerunners of insects possessed a twentieth segment, the telson, but it was lost at some point during their evolution. While the development of xastres is known in less detail, there is enough understood about it to recognize some significant differences. The head of the xastre seems to be formed of only five somites, and the ancestral telson, rather than disappearing, instead merged with the rear two segments of the proto-xastre's abdomen to form the xastre's araea. The most immediately obvious outward sign of the different formation of the tagmata is that insects have legs attached only to the thorax, whereas xastres have legs on both the sternon and the araea.
Varieties
There are at least thirteen known species of xastre, divided into three genera: the type genus Tritholus and the genera Arigonophorus and Phalangiomimus. The most abundant species of xastre is the common xastre, Tritholus vulgaris, though it is common only relative to other xastre species. Also called the "rugose xastre" after its cockled carapace, this species typically reaches about four meters in length. The largest xastre species is the marginal xastre, Tritholus antygus, but the most aggressive is the great white xastre, T. anthrophonus, so named not for its color but for the fact that it's found primarily in the White Main. (The similarity of its name to that of the great white shark Carcharodon carcharias is coincidental.)
Other well-known species include the brightly colored jewel xastre, Phalangiomimus iridus; the venomous pernicious xastre, Arigonophorus loegius; and the shelf xastre, P. crocales, a relatively small species notable for its ability to survive for some time out of water and its habit of venturing onto land to hunt.
Ecology and behavior
Xastres are apex predators, positioned at the top of their food chains. They possess an intelligence comparable to that of a mammalian predator such as a tiger or a wolverine, and often seem to show remarkable cunning. For the most part, xastres are solitary creatures, and very rarely is more than one seen at a time. (The wolf xastre, Arigonophorus cynegesii, is an exception; the smallest known species of xastre, these are unique in being pack hunters.) Most species of xastre specialize in hunting animals about half their (linear) size, though they frequently take on larger prey if they see some circumstance that gives them an advantage, and may resort to smaller prey if hungry and desperate. Notably, this puts humans in the size range of the preferred prey of most xastre species, and indeed there have been numerous documented cases of xastres attacking humans—and no doubt would be many more if it weren't that humans rarely venture into the oceanic depths where xastres are found. Xastres also pose a significant threat to human-sized organisms that are more commonly found in the ocean, such as bulurangs and aquanauts.
Like other arthropods, xastres undergo regular molting, discarding their old carapaces to make room for growth and forming new, larger carapaces to replace them. During its teneral period, the soft-shelled xastre is especially vulnerable and may find a suitable crevice to hide in, although their quirites do not molt at the same time as the rest of their carapce, so even then they are not entirely defenseless. The molt of the quirites occurs just after the rest of the molt has completed, and occurs to one quiris at a time, so the xastre is never without one quiris with which to hunt prey or defend itself. (Usually it is the left quiris that molts first, though there doesn't seem to be any particular reason for this.) The cast-off shells of xastres form a significant resource exploited by other marine organisms, including bone stars and twiddleworms; while the xastre's cuticles are heavily mineralized, they do have some protein content, and their sheer size ensures they contain enough nutritive value to sustain colonies of such conchophages. The coneslug makes its home in a discarded xastre tergum, and there's a rare sort of undead, the spindlegast, that exclusively inhabits and animates xastre exuviae.
Reproduction
Xastre reproduction is initiated by the fertile male releasing its sperm into the surrounding water. The female absorbs it through a posterior vaginal orifice called a gonotreme, and the sperm is stored in the spermatheca until it's needed to fertilize eggs. The male and female xastres do not remain associated after the fertilization process; indeed, the male may leave immediately after releasing its sperm and before the female has even taken it in. About four weeks later, the female lays clusters of eggs. These clusters are coated in hard shells and look like irregular rocks; the juvenile xastres are able to release special chemicals to dissolve the shells from the inside. The mother does not stay with the eggs, and the xastre hatchlings are on their own, although even a newly hatched xastre is still a match for most creatures it's likely to encounter.
Ametabolous, xastre do not change their overall shape as they grow; a xastre hatchling has the same basic form as an adult. Some of their relative proportions do change slightly, however; juveniles do tend to have proportionately larger heads and thinner legs than adult xastres. In many species, the color also changes as they grow, as may the texture or pattern of the cuticle.