5io 



PHYLOGENY OF THE CRUSTACEA. 



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 indepen- 

 dent derivation of the Cirripedia from some early bivalve Phyllopod form. 



Ostracoda. 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 



B 



D 



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, a. archenteron ; 

 b. blastopore ; ms. mesoblast ; ec. 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 ; 

 ec. epiblast ; en. hypoblast. 



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

 y. yolk ; n. nucleus ; /. pseudopodial process. 



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

 n. nuclei. 



