. Text book of zoology. Zoology. 222 Arthropoda. Class 1. Crustacea. Decapods the zoaea is followed by the "mysia-stage" (so-called on account of its resemblance to tbe perfect Mysis), in wbich the development of the ambulatory limbs is completed, and the animals swim actively by means of their exopods and the posterior maxillipeds; the swimmerets are still absent, or only imperfectly developed. This stage is followed by the p r a w n-stage, in which the exopods of the ambulatory limbs atrophy, whilst the swimmerets are strongly developed, and form powerful swimming-organs, enabling
. Text book of zoology. Zoology. 222 Arthropoda. Class 1. Crustacea. Decapods the zoaea is followed by the "mysia-stage" (so-called on account of its resemblance to tbe perfect Mysis), in wbich the development of the ambulatory limbs is completed, and the animals swim actively by means of their exopods and the posterior maxillipeds; the swimmerets are still absent, or only imperfectly developed. This stage is followed by the p r a w n-stage, in which the exopods of the ambulatory limbs atrophy, whilst the swimmerets are strongly developed, and form powerful swimming-organs, enabling the animal, which in this, as well as the foregoing stages, is entirely or almost transparent, to move actively about in the surface waters. For one division of the Decapoda, the Natantia, the prawn-stage corresponds with the adult; they remain throughout life in this state, the. Fig. 184. Myaia-stage of Penwvs (enlarged). I—F the five ambulatory limbs with long exopoda and short endopods.—After Claua. swimmerets being the permanent natatory-organs ; in another division, the Reptantia, the prawn-stage is not permanent, but after some time, the abdominal limbs, with the exception of the sixth pair, atrophy J they cease to be swimming-organs, the animal becomes opaque, and the power of swimming is lost. Some Decapoda (, the Lobster), hatch at the mysis-stage, and consequently only pass through this and the prawn-stage. Others (, Crahs) pass direct from the Zoaea to the prawn, missing out the mysis-stage, , at no time are the ambulatory limbs provided -with swimming rami. The Decapoda constitute a very large group, including the largest forms among the Malacostraca. Most of the species are marine; a comparatively small number (Cray-fish, some Prawns) are fresh- water; whilst a few are terrestrial. Sub-Order 1. Natantia {Prawns). The skeleton of the Prawns is not very hard, it is merely horny; the animal is transparent or semi-transparent; the body is compressed (Pi
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
