VIII 



AETHEOPODA 



213 



This stage is represented by the Zoaea larvae of Caridea, 

 Anomura, and Brachyura. The Zoaea is transformed by several 

 moults; lirst into a Metazoaea, in which the rudiments of the 

 appendages of the abdomen and of the hinder segments of the 

 thorax appear; secondly into a so-called "Mysis" stage, in which, 

 typically, the hinder segments of the thorax bear forked limbs 

 designed to assist in swimming, and in which the first appendages of 

 the thorax tend to become somewhat diminished in size and degraded 

 from the rank of locomotor organs of prime importance to that of 

 maxillipedes. 



This Mysis larva, as its name implies, is of such obvious ancestral 

 significance that no one has ever attempted to deny that it represents 

 a Schizopod ancestor. We can, however, even here trace the work 



at 



FIG. 160. " Mysis " larva of Homrtrtis mnericanus, lateral view. (After Herrick.) 

 r.i-'-i'.! 1 ", the seven exopodites borne by seven of the thoracic legs. 



of the same modifying tendencies which have, obscured the ancestral 

 significance of the Nauplius and the Zoaea. In such of the Nephrops- 

 idea as do not complete their development within the egg-shell (cf. 

 Ho mar us, the lobster, and Nephrops, the Norway lobster) the larva 

 emerges in the Mysis condition, with this difference, that the 

 abdominal appendages are at first quite suppressed. 



In the Loricata, of which the rock-crayfish, Pali H in- HA, and the 

 square-nosed lobster Scyllarus are the best known representatives, 

 the larva emerges from the egg-shell in a singularly modified Mysis 

 stage (Fig. 161). In this larva the thorax is broad and flat and of a 

 glassy transparency ; the abdomen, though distinctly divided into 

 segments, is very small and has no rudiments of appendages. The 

 thorax has only six of the eight pairs of appendages which it should 

 possess if normally developed, and of these, those representing the 

 first two pairs of maxillipedes are small but those represent ing the 

 third maxillipede and the first three pairs of walking thoracic 



