Amiskwia sagittiformis Very few specimens of this organism have been found, only eighteen in the Burgess shale — which may be a reflection of its genuine rarity, but is more likely to be due to taphonomic (preservational) or behavioural factors. The fossils reach 25 mm (1 in) in length. The head is rounded, tipped with two tentacles, and appears to contain a four-ganglion brain; the body flattens out and broadens in the trunk, which appears to have been fairly muscular. Where the trunk meets the head there is a small tubular opening, which can be interpreted as the mouth; the gut terminates where the trunk narrows and meets the tail, which is broad and paddle shaped. The body morphology suggests a free energetic swimmer, which may be consistent with the dearth of fossils. Amiskwia was originally categorized by paleontologist Charles Walcott. Walcott thought he saw three buccal spines in the fossils, and therefore categorized Amiskwia as a chaetognath worm (arrow worm). However, Amiskwia appears to lack the characteristic grasping spines and teeth of other Burgess fossil arrow worms. Later scientists suggested an affinity with the nemerteans (ribbon worms), but the evidence for this was somewhat inadequate. Conway Morris, on re-examining of the Burgess Shale fauna in the 1970s, described it as being the single known species in an otherwise unknown phylum, given that it has two tentacles near its mouth, rather than the characteristic single tentacle of true nemerteans. (Nemerteans do not have a single tentacle. However, a pair of antero-lateral tentacles is present in 2 of the many genera of pelagic nemerteans. Nemerteans do have a single eversible - normally internal - proboscis, which when everted could resemble an anterior median tentacle if fossilized. Whether retracted or everted, the proboscis is the only structure in pelagic nemerteans likely to fossilize, as it is the only structure with substantial connective tissue and muscle. The body wall has almost no muscle or connective tissue and is exceedingly unlikely to fossilize; hence, a pelagic nemertean fossil would be only the proboscis). Butterfield implies from the appearance of the fossils that the organisms may have lacked a cuticle: whilst this is also true of the nemerteans, these organisms lack a coelom and are thus unlikely to fossilise. He goes on to argue that the absence of cuticle is characteristic of the Chaetognaths; whilst teeth would be expected, a similar fossil, Wiwaxia, only shows such structures in 10% of the expected instances, and Anamolocarids are often found detached from their mouthparts, so the absence may be taphonomic rather than genuine. The absence of spines could simply mean that the fossils represent young organisms — or that later chaetognath evolution involved paedomorphosis. While more work on undescribed specimens is required before firm consensus is reached, the discovery of P. spinosa, which strongly resembles modern Chaetognaths, in sediments of a similar age in Chengjiang biota places doubts on its position in the Chaetognath crown group. It has also been likened to the molluscs, and bears limited resemblance to unusual forms such as Pterotrachea. Of course, many similarly enigmatic Cambrian fossils probably represent stem groups to living taxa, and there is no reason to assume that this organism must fall into an extant crown group at all. The name Amiskwia sagittiformis derives from the nearby Amiskwi River, and its sagittiform shape.