Outlines of zoology . wn as the ventralplate, the cells begin to thicken ; at this region an invagination occurs,which represents the gastrula. At the anterior lip of the blastopore themesoderm appears, being many-celled from the first. Soon the blasto-pore closes ; the cavity of the gastrula thus becomes a closed sac—thefuture mid-gut. The cells of this archenteron take up the core of yolk CRA YFISH. 295 into themselves in a way which early suggests their future digestivefunction. On the surface of the. egg there have already appearedectoderpiic thickenings,—the so-called eye-folds,-—rudiment
Outlines of zoology . wn as the ventralplate, the cells begin to thicken ; at this region an invagination occurs,which represents the gastrula. At the anterior lip of the blastopore themesoderm appears, being many-celled from the first. Soon the blasto-pore closes ; the cavity of the gastrula thus becomes a closed sac—thefuture mid-gut. The cells of this archenteron take up the core of yolk CRA YFISH. 295 into themselves in a way which early suggests their future digestivefunction. On the surface of the. egg there have already appearedectoderpiic thickenings,—the so-called eye-folds,-—rudiments of theappendages, and of the thoracic and abdominal regions. In the later stages invaginations of the ectoderm form the fore- andhind-gut, which grow inward from opposite ends to meet the endo-dermic mid-gut. The ear-sac and the greater part of the gills have? also an ectodermic origin. From the mid-gut the digestive gland isbudded out. The heart, the blood vessels, blood, and muscles are dueto the Fig. 148.—Longitudinal section of later embryo ofAstacus.—After Reichenbach. Ec, Ectoderm; vi., mesoderm cells; ; cerebral ganglia; st.^stomodseum; A., anus; 7., telson;^., ventral ganglia; ,sternal sinus ; pd., proctodeum; A., heart; tng., mid-gut; yolkpyramids dark. As usual, the nervous system arises from an ectodermic eye arises partly from the optic ganglia of the brain, partly fromthe eye-folds, and partly from the epidermis. When the young crayfishes are hatched from the egg-shells, they stillcling to these, and thereby to the swimmerets of the mother. In mostrespects they are like the adults, but the cephalothorax is convex andrelatively large, the rostrum is bent down between the eyes, the tips ofthe claws are incurved and serve for firm attachment, and there areother slight differences. The noteworthy fact is that the developmentis completed within the egg-case, and that it is continuous withoutmetamorphosis. The shortened life
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
