CRUSTACEA. 425 



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 Claxis' view of the Nauplius) would seem to be im- 

 plied 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 centro- 

 lecithal 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 Palsemon 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, Palsemon, 

 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 and is finally 

 obliterated. Very shortly after this occurrence there is formed, 

 slightly in front of the point where the last trace of the blasto- 

 pore was observable, a fresh epiblastic invagination, which gives 

 rise to the proctodseum, and the opening of which remains as 

 the definite anus. The proctoclaBum (fig. 238 A, hg} is very soon 

 placed in communication with the mesenteron (mg). The stomo- 

 daeum (fg) is formed during the same stage as the proctodseum. 

 It gives rise to the oesophagus and stomach. The hypoblast cells 

 which form the wall of the archeuteron grow with remarkable 



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

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



