. The Cambridge natural history. Zoology. VI PENEIDEA AND CARIDEA SHRIMPS AND PRAWNS I 59 are described by Alcock ^ as possessing peculiar secondary sexual characters. Thus Parapene^is rectaciUus 6 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. As
. The Cambridge natural history. Zoology. VI PENEIDEA AND CARIDEA SHRIMPS AND PRAWNS I 59 are described by Alcock ^ as possessing peculiar secondary sexual characters. Thus Parapene^is rectaciUus 6 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. 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.^ 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 meta- morphosis which is primitive in respect to the order of forma- tion of the segments from before backwards. The larva hatches out as a Nauplius (Fig. 106), which by tlie orderly addition of segments ' Loc. cit. p. 150. ^ Keeble and Gamble, Phil. Trans., Ser. B, cxovi., 1904, p. 295. The cliromato- phores are also directly responsive to light, but the lasting adaptations to colour- backgrounds are brought about indirectly, the stimulus being transmitted through the eyes and nervous System. The influence of liglit may also art'ect the metabolism of the animal, the chromatophores beiug accompanied by a ramifying fatty tissue, which disappears if the animal is kept in the Fig, 106.—Nauplius larva of Peneus, sp. x 25. (From Balfour, after F. Jluller).. Please note that these images are extr
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