172 



SYSTEMATIC ZOOLOGY 



are often more or less covered with hydroids or sea anemones 

 (Figs. 47 and 61), which serve the same purpose and derive the 

 same advantage. This superficial way of living together is 

 often called commensalism. 



The development of the Decapoda is attended by a consider- 

 able variety of larval types. In some fresh-water species there 

 is no free-swimming larva, but the nauplius remains within the 

 eggshell and is there metamorphosed into a form like the adult. 



Fn;. 165. Larvae of crabs. A, zoea stage of Maya; R. megalops stage of Por tun us. a%- 

 a G , abdominal segments ; h, heart ; /, antennule ; 2, antenna ; I-VI 1 1, thoracic appendages. 

 (Aftei ( llaus, from Parker and Haswell's Text-book.) 



In some prawns the larva leaves the egg as a nauplius, just 

 as in the Entomostraca. Then by successive moults or ecdvses 

 it passes into a form called the zoea larva (Fig. 165). This is 

 succeeded by a stage resembling the adult Schizopoda and 

 called the Mysis stage, in which it is provided with biramous 

 thoracic appendages. This stage is followed by the adult form. 

 In the crabs the nauplius remains in the egg membrane and the 

 zoea is the first free-swimming larva. It is succeeded by a stage 

 called the megalops larva (Fig. 165), which in turn is metamor- 

 phosed into the form of the adult crab. In the lobster the only 

 free-swimming larva is the Mysis stage, as the embryo passes 

 through both the nauplius and the zoea stages in the egg. 



