yo COMPARATIVE ANATOMY AND PHYSIOLOGY. 



twenty segments to their body, and all but one of 

 these bear appendages ; as many as six may be con- 

 verted into 

 gnathites, and 

 the larvae ordi- 

 narily, though 

 not always, are 

 set free at a 

 later than the 

 nauplius stage. 



l.The Pod- 

 optitlialmata 

 are so called 

 from the fact 

 that their eyes 

 are placed on 

 stalks (Fig. 29); 

 in them some of 

 the dorsal por- 

 tions of the 

 thoracic m eta- 

 meres take part 

 in the formation 

 of a carapace. 

 Such are cray 

 fishes, lobsters, 

 shrimps, and 

 crabs. 



2. The He- 

 d ra op lit li al- 

 ma ta have the 

 eyes sessile, and 

 no carapace is 

 developed ; the 

 A in phi pod a (e.g. sandhopper) are the least modi- 

 fied ; some of the Isopoda (such as the wood-louse) 

 are fitted to and do dwell on land, while the 



Fig. 30. -Limulus moluccanus. 



