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 (e.g., the Lobster), hatch at the mysis-stage, and consequently 

 only pass through this and the prawn-stage. Others (e.g., Crahs) pass direct 

 from the Zoaea to the prawn, missing out the mysis-stage, i.e., 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 (Pig. 182) ; the abdomen strong and curved, and 



