. Elementary text-book of zoology. Zoology. 476 CRUSTACEA. are usually inserted externally and somewhat ventrally to the first pair on a flat plate placed in front of the inouth (epistom or oral shield) : they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance with 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, is absent in many prawns (Candida?). They are either straight and strongl
. Elementary text-book of zoology. Zoology. 476 CRUSTACEA. are usually inserted externally and somewhat ventrally to the first pair on a flat plate placed in front of the inouth (epistom or oral shield) : they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance with 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, is absent in many prawns (Candida?). 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 (Palmnonidw and Alpliddce). The anterior ruaxillse always consist of two lamella? and a palp, which is usually simple. The posterior maxilla-, on which there are usually four lamellae (two double lamella) 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 tive pairs of thoracic appendages for use as legs; of these the two last are sometimes re- duced or may even be ^ entirely absent (Leuci- fer) as the result of FIG. 372. -Young form (larva) of the lobster (after G. retrogressive changes. O. Sars). K, rostrum; A', A", antennte; A""', third rrn ,1 maxilliped ; F 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 ventral 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 JJecapoda leave the egg membranes in the zorea form; in ffomarus, amongst the Macrura, the metamorphosis is much reduced and the just-
Size: 2228px × 1121px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
