206 



CEUSTACEA. 



adductor muscle (SM), it already shows a typical Ostracodan feature, 

 the development of -which has evidently been shifted back second- 

 arily to this early stage. Further metamorphosis takes place, as in 

 the Phyllopoda, by the gradual development in regular order of the 

 posterior pairs of limbs, the growth of the posterior section of the 

 body and of the corresponding internal organs. In accordance with 

 these changes in the body, a change in the shell (observed by Zenker, 

 Xo. 35) takes place. In the NaiqMus, the anterior half of the shell 

 is remarkably developed. Its greatest height, as well as its greatest 



/: (Mxl i Mx 



Fig. 100.— Two further stages of development of Cypris (after Claus, from Balfour's 7'ext- 

 book). ^, fourth stage. U, fifth stage. Jlfx-', first maxilla; il/.r" and ^', second maxilla ; /", 

 first pair of feet ; L, liver. 



breadth, lies anteriorly to the middle of the body. The anterior 

 section of the shell appears sharply inclined, while the posterior end 

 of the shell runs out more gradually to a point. As the posterior 

 part of the body develops, the posterior halves of the shell also 

 increase in size (Figs. 99-101). An exception to the rule that the 

 limbs appear in order from before backward is found in the second 

 maxillae which are somewhat retarded, not appearing until after the 

 limbs which lie posteriorly to them have developed. There is here 

 a recurrence of a feature found in the Phyllopoda and a few Copepoda 

 (Ci/dojndae, Harpadidae). 



