476 CEUSTACEA. 



are usually inserted externally and somewhat ventrally to the first 

 pair on a flat plate placed in front of the mouth (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 (Candidas). 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 Alplieidce}. The anterior maxillae 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 

 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- 

 f&r) as the result of 



Fio. 372. -Young form (larva) of the lobster (after G. retrogressive changes. 

 O. Bars). E, rostrum; A', A", antennae; K"'. third mi ,-. 

 maxilUpedjJF' anterior ambulatory leg. Tlie thoracic 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 Decapoda leave 

 the egg membranes in the zoaea form ; in Homarus, amongst the 

 Macrura, the metamorphosis is much reduced and the just-hatched 

 young possesses all the thoracic legs, which are, however, provided 

 with external swimming rami, but it is still without the abdominal 

 feet (fig. 372). 



Embryonic development. In addition to the classical researches 



of Rathke * on the crayfish, more recent works, especially those of 



* Besides Rathke 1. c. and Lereboullet 1. c., and a Russian paper of Bobretzky, 



