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. [Illustration: FIG. 107.—Protozoea larva of _Peneus_, sp. × 25. (From Balfour, after F. Müller.) ] [Illustration: FIG. 108.—Zoaea larva of _Peneus_, sp. × 25. _A_, _A′_, 1st and 2nd antennae; _Ab.6_, 6th abdominal appendage; _Mxp_, 2nd maxillipede; _T_, 4th–8th thoracic appendages (future walking legs). (After F. Müller.) ] Some of the Peneid larvae take on very peculiar forms, _e.g._ the Zoaeae of the Sergestidae,[134] which often develop the most wonderful spines all over the body. [Illustration: FIG. 109.—Mysis stage in the development of _Peneus_, sp. _A.2_, 2nd antenna; _Ab.6_, 6th abdominal appendage; _T_, telson; _Th_, the biramous thoracic appendages. (After Claus.) ]