424 



PHYLOGENY OF THE CRUSTACEA. 



very primitive and general, and the number (without taking into considera- 

 tion the doubtful case of Cryptophialus) does not correspond to that in 

 Copepoda. On the other hand the paired eyes and the bivalve shell 

 form great difficulties in the way of Glaus' view. It is clear that the 

 Cypris stage represents more or less closely an ancestral form of the 

 Cirripedia, and that both the large bivalve shell and the compound eyes 

 were ancestral characters. These characters would seem incompatible 

 with Copepod affinities, but point to the independent derivation of the 

 Cirripedia from some early bivalve Phyllopod form. 



Gstracoda. The independent origin of the Ostracoda from the main 

 Crustacean stem seems probable. Claus points out that the Ostracoda 

 present by no means a simple organisation, and concludes that they were 

 not descended from a form with a more complex organisation and a larger 

 number of appendages. Some simplifications have however undoubtedly 

 taken place, as the loss of the heart, and of the compound eyes in many 

 forms. These simplifications are probably to be explained (as is done by 

 Claus) as adaptations due to the small size of body and its enclosure in a 

 thick bivalve shell. Although Claus is strongly opposed to the view that 



en 





\ 



v 



FIG. 237. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From Parker; 

 after Reichenbach.) 



A. Section through part of the ovum during segmentation, n. nuclei; w.y. white 

 yolk; y.p. yolk pyramids; c. central yolk mass. 



B and C. Longitudinal sections during the gastrula stage. .. archenteron ; b. blas- 

 topore; ms. mesoblast; cc. epiblast; en. hypoblast distinguished from epiblast by 

 shading. 



D. Highly magnified view of the anterior lip of blastopore to shew the origin of 

 the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast- 

 cc. epiblast; en. hypoblast. 



E. Two hypoblast cells to shew the amceba-like absorption of yolk spheres. 

 y. yolk; n. nucleus; p. psendopodial process. 



F. Hypoblast cells giving rise endogeuously to the secondary mesoblast (s.ms.). 

 n. nuclei. 



