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 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 Claus’ 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 Palæmon 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, Palæmon, [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[208], and the sack itself then forms the mesenteron.
Fig. 238. Two longitudinal sections of the embryo of Astacus. (From Parker; after Bobretzky.)
A. Nauplius stage. B. Stage after the hypoblast cells have absorbed the food-yolk. The ventral surface is turned upwards. fg. stomodæum; hg. proctodæum; an. anus; m. mouth; mg. mesenteron; abd. abdomen; h. heart.
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 blastopore was observable, a fresh epiblastic invagination, which gives rise to the proctodæum, and the opening of which remains as the definite anus. The proctodæum ([fig. 238] A, hg) is very soon placed in communication with the mesenteron (mg). The stomodæum (fg) is formed during the same stage as the proctodæum. It gives rise to the œsophagus and stomach. The hypoblast cells which form the wall of the archenteron grow with remarkable rapidity at the expense of the yolk; the spherules of which they absorb and digest in an amœba-like fashion by means of their pseudopodia. They become longer and longer, and finally, after absorbing the whole yolk, acquire a form almost exactly similar to that of the yolk pyramids during segmentation ([fig. 238] B). They enclose the cavity of the mesenteron, and their nuclei and protoplasm are situated externally. The cells of the mesenteron close to its junction with the proctodæum differ from those elsewhere in being nearly flat.