These are remarkably archaic animals of great rarity, though they were common enough in Triassic seas, and have come down to us as fossils from those times, being thus among the oldest Decapoda known. They only survive now as deep sea species, and the genus discovered by the _Challenger_,[131] _Willemoesia_ (Fig. 105), confirmed the expectations of the _Challenger_ naturalists that the abysses of the ocean would contain relics from older periods which had managed to survive where the competition was not so keen. The genus _Willemoesia_ is very widely distributed, being dredged up from below a thousand fathoms in the Indian Ocean, the Mediterranean, North and South Atlantic, and the Pacific oceans. All the walking legs are chelate, and the animal is quite blind, as are all the Eryonidea, the eye-stalks being fused with the carapace. Only a single family =Eryonidae= is recognised. =Tribe 3. Peneidea.—Tribe 4. Caridea.= We will now consider the Shrimps and Prawns, since in them occurs the most complete metamorphosis found in the Decapoda. The Peneidea are distinguished from the ordinary Prawns and Shrimps (Caridea) by having the first three instead of the first two pereiopods chelate. The genus _Peneus_ affords several species which are of commercial value as objects of food; the edible Prawns of the Mediterranean belong to this genus, while in the North Sea two of the Caridea, viz. the Shrimp, _Crangon vulgaris_, and the Prawn, _Palaemon serratus_, are the forms very commonly eaten. Both subdivisions are well represented in the deep sea fauna from all parts of the world. _Glyphocrangon spinulosa_ (Fig. 110, p. 164) is a deep sea Shrimp with eyes that have lost their pigment, and with the body covered with spines, while the last abdominal segment is fused with the telson to form a sharp bayonet-like process at the hind end of the body. Some of the deep-sea Prawns of the Indian Ocean are described by Alcock[132] as possessing peculiar secondary sexual characters. Thus _Parapeneus rectacutus_ ♂ has one lash of the first pair of antennae peculiarly bent to form a clasping organ, while _Aristaeus crassipes_ has a hook on the end of the third maxillipede. In the latter the females have much longer rostra than the males, and are in general more powerfully built, so that they seem to have usurped the proper functions of the male, and probably engage in combats with one another over his person. [Illustration: FIG. 106.—Nauplius larva of _Peneus_, sp. × 25. (From Balfour, after F. Müller). ]