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.
Fig. [106].—Nauplius larva of Peneus, sp. × 25. (From Balfour, after F. Müller).
As a general rule the Shrimps and Prawns occur in large shoals in the shallow waters of the littoral zone, and they have a remarkable power of adapting their colours to the surroundings in which they happen to be at any particular moment.[[133]] This is brought about by the variously coloured chromatophores, which contract and expand in obedience to a stimulus transmitted through the eyes of the animal. A number of the Palaemonidae go up rivers into fresh water, while one family, the Atyidae, live in the completely fresh water of rivers and inland lakes. The Peneidea undergo a very complete metamorphosis which is primitive in respect to the order of formation of the segments from before backwards. The larva hatches out as a Nauplius (Fig. [106]), which by the orderly addition of segments behind is converted into the Protozoaea (Fig. [107]), possessing two pairs of biramous maxillipedes. It should be noted that the maxillae, which are foliaceous in the adult, are laid down in this condition in the larva, and this principle holds good throughout Crustacean metamorphosis, viz. that when a limb is foliaceous in the adult it is foliaceous in the larva, and when biramous in the adult it is biramous in the larva. Whilst the rest of the thoracic limbs are still rudimentary, the sixth pair of pleopods are being precociously developed (Fig. [108]), being the only precociously formed limbs in the Peneidea, though the abdominal segments are fully marked off before the thoracic segments, and so must be considered as precocious in development. When the biramous thoracic limbs are completed the abdominal biramous pleopods are added, beginning from in front backwards. Thus the Mysis stage (Fig. [109]) is reached, which resembles in all particulars the adult condition of the Schizopoda. The adult Prawn develops from this stage by the loss of some or all of the exopodites on the thoracic pereiopods.
Fig. [107].—Protozoea larva of Peneus, sp. × 25. (From Balfour, after F. Müller.)
