300 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



great stalks, the antennules, and the antenna?, at its extrem- 

 ity, separating them from the rest of the cephalo-thorax, 

 which is covered by a delicate carapace, bent down at the 

 sides. The anterior thoracic members are rudimentary, and 

 the posterior pair is absent. The heart is short and rounded, 

 and situated, as usual, in the thorax. 



It has been seen that in Astacus fluviatilis, as in Limu- 

 lus and Daphnia^ the embryo slowly and gradually passes 

 into the form of the adult ; to which it is so similar when it 

 leaves the ^^^^ that the changes of the young present noth- 

 ing comparable to the well-known metamorphoses of Butter- 

 flies and Beetles. 



But most Podoplitlialmia rather resemble the Copepoda 

 and the majority of the Entomostraca,, in the fact that the 

 young, when they leave the e^^, have a totally dissimilar 

 form to that of the parent, and only acquire the adult con- 

 dition after a series of ecdyses. 



The observations of Fritz Miiller ' have shown that the 



Fig. "^l.—Peneus.—A^ JVauplius-siaf^e. B, Zocea or Copepod stage. C, ScMzopod- 

 stage. (After Muller.) 



young of a species of Prawn (Peneus) undergo a metamor- 

 phosis which runs j^arallel with that of the Cojwpoda. When 

 it leaves the egg (Fig. 77, A), the young Peneus has an 



1" Fur Darwin," 18G4 



