. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 476 CBUSTACEA. are usually inserted externally and somewhat ventrally to the first pair on a flat plate placed in front of the mouth (ejnstom or oral .shield): they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance Avith a pore at its end, through which the duct of the antennal gland (green gland) opens. The mandibles vary considerably in shape in the different forms, but have, as a rule, a two or three-jointed palp, which, however, Ls absent in many pr
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 476 CBUSTACEA. are usually inserted externally and somewhat ventrally to the first pair on a flat plate placed in front of the mouth (ejnstom or oral .shield): they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance Avith a pore at its end, through which the duct of the antennal gland (green gland) opens. The mandibles vary considerably in shape in the different forms, but have, as a rule, a two or three-jointed palp, which, however, Ls absent in many prawns (Carididse). They are either straight and strongly toothed on their thickened anterior edge {Brachyura), or are slender and much bent [Crangon), or else forked at the ends {Palcemonidce and Aljiheidce). The anterior maxillae always consist of two lamellaj and a palp, which is usually simple. The posterior maxillse, on which there are usually four lamellne {two double lamellfe) as well as palps, bear a large respiratory plate with setose edges (scaphognathite). These are followed by three pairs of maxillipeds, which, as a rule, have a flagellate appendage. There remain, therefore, only a five pairs of thoracic appendages for use as legs; of these the two last are sometimes re- duced or may even be entirely absent (Leuci- /er) as the result of Fig. 372.—Young form (larva) of the lobster (after G. retrogressive changes. O. Sars). fi. rostrum; ^', ^", antennoe; r'". third rpj thoncic secments maxilliped; J" anterior ambulatory leg. ^'^^^ tnoracic segments to which the ambulatory legs belong are, as a rule, all or all but the last fused together and form on the venti-al side a continuous plate, which in all the Brachyura is broad. The legs consist of seven joints, which corre- spond to those of the Arthrostraca, and frequently end with a chela or prehensile hand. Development.—The greater number of marine Decajwda leave the e^g membranes in the zoaea form; in
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
