CRUSTACEA. 511 



the number of the appendages has been reduced, yet the very fact of the 

 (in some respects) complex organisation of this group might seem to indicate 

 that it cannot have diverged from the Phyllopod stem at so early a stage as 

 (on Glaus' view of the Nauplius) would seem to be implied by the very small 

 number of appendages which is characteristic of it, and it therefore appears 

 most probable that the present number may be smaller than that of the 

 ancestral forms. 



The formation of the germinal layers. 



The formation of the germinal layers has been more fully 

 studied in various Malacostraca, more especially in the Decapoda, 

 than in other groups. 



Decapoda. To Bobretzky (No. 472) is due the credit of 

 having been the pioneer in this line of investigation ; and his 

 researches have been followed up and enlarged by Haeckel, 

 Reichenbach (No. 488), and Mayer (No. 482). The segmentation 

 is centrolecithal and regular (fig. 237 A). At its close the 

 blastoderm is formed of a single uniform layer of lens-shaped 

 cells enclosing a central sphere of yolk, in which as a rule all 

 trace of the division into columns, present during the earlier 

 stages of segmentation, has disappeared ; though in Palaemon 

 the columns remain for a long period distinct. The cells of the 

 blastoderm are at first uniform, but in Astacus, Eupagurus, 

 and most Decapoda, soon become more columnar for a small 

 area, and form a circular patch. The whole patch either 

 becomes at once invaginated (Eupagurus, Palaemon, fig. 239 A) 

 or else the edge of it is invaginated as a roughly speaking 

 circular groove deeper anteriorly than posteriorly, within which 

 the remainder of the patch forms a kind of central plug, which 

 does not become invaginated till a somewhat later period 

 (Astacus, fig. 237 B and C). After the invagination of the 

 above patch the remainder of the blastoderm cells form the 

 epiblast. 



The invaginated sack appears to be the archenteron and its 

 mouth the blastopore. The mouth finally becomes closed 1 , and 

 the sack itself then forms the mesenteron. 



In Astacus the archenteron gradually grows forwards, its 

 opening is at first wide, but becomes continuously narrowed 



1 Bobretzky first stated that the invagination remained open, but subsequently 

 corrected himself. Zeit. f. Wiss, Zool., Bd. xxiv. p. 186, 



