. The theory of evolution in the light of facts. seecertainly also, in the adult form, still other true crab-like characters, as for instance in the constructionof the mouth, the nervous system, the legs, etc. Inshort there remains everything of a crab type : in thefirst place the embryogeny up to that stage where suchconstructions were added, the positive adaptations to thesessile mode of life, and furthermore all the charactersof the Cirripedia, which also can be of service in theirold form in the new mode of life—jaws, nerve system,and make of the legs. In other cases certainly it is •^ Hef
. The theory of evolution in the light of facts. seecertainly also, in the adult form, still other true crab-like characters, as for instance in the constructionof the mouth, the nervous system, the legs, etc. Inshort there remains everything of a crab type : in thefirst place the embryogeny up to that stage where suchconstructions were added, the positive adaptations to thesessile mode of life, and furthermore all the charactersof the Cirripedia, which also can be of service in theirold form in the new mode of life—jaws, nerve system,and make of the legs. In other cases certainly it is •^ Heffe : Abstammungslehre und Darwinismus, p. 30. 206 THE THEOEY OF EVOLUTION only the embryonic development which now gives aclue to the systematic relationship—for instance withSacculina carcini, the female of which finally becomesan egg-bag which pervades the whole body of its hostanimal (a crab) with a cotton-hke web. The embryonicdevelopment on the other hand is that of the Cirripedia(Figs. 40 and 41). From all this we conclude, and, as. Fig. 40.—Cirripedia. SaccuUni carcini {afterBelage). 1. Second Nauplius stage. 2, 3. Thesame after attachment of the breast piece andloss of tail. 4. The bottle-like stage passinginto the interior of the crab. 5. The finalstage. Letter reference: an, antennae; au, eye ; c,tail; cp, anus ; kr, main shield of the crab ; r,the cell tube of the Sacculina larva penetratingthe host; s, Sacculina externa; z, central cellmass. {After von Graff.) it appears to us, with entire right, that the Lepas specieswere formerly free-swimming crabs which subsequentlyadopted a sessile existence. Their embryogeny aloneand the still remaining crab characters show us clearlythe true nature of the Barnacles. The whole groupof the Cirripedia behaves in a similar manner. Veryremarkable beings are also the animals known asParasitica which as parasites live especially on fish,upon whose skin or gills they attach themselves by EMBKYOGENY 207 suckers. They were form
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