THE DEC A POD A 279 



and in those families of the Brachyrhyncha formerly grouped 

 together as Cyclometopa, it suffers reduction in many of the Cato- 

 metopa, especially in terrestrial and parasitic forms, and in the 

 majority of the Oxystomata. 



The table on p. 280 gives the branchial formulae in a series of 

 representative forms. 



The arrangements for maintaining a current of water through 

 the branchial chamber and for preventing the ingress of foreign 

 particles are very varied and often complex. The branchial 

 current is caused by the vibratory movements of the scaphognathite 

 or exopodite of the maxilla, and as a rule it sets from behind 

 forwards, though it appears that in some cases, especially in 

 Decapods which burrow in sand or mud, the direction of the 

 current is periodically reversed. In the simplest cases, as in most 

 of the Macrurous groups, the water enters along the lower margin 

 of the branchial chamber, which is protected by setae, and in par- 

 ticular by those of the setobranchiae. The exhalent current passes 

 out at the sides of the oral region in front. This arrangement is 

 modified in the Brachyura by the free edge of the branchiostegite 

 fitting closely to the bases of the legs on each side, only leaving an 

 aperture for the ingress of water in front of the base of the cheliped. 

 This aperture is provided with an opercular valve formed by the 

 base of the long epipodite of the third maxilliped. These arrange- 

 ments may be further complicated in various ways, especially in 

 the case of burrowing forms. The exhalent passages, which in 

 some cases may by reversal of the current become inhalent, are not 

 unfrequently prolonged towards the front of the head by the apposi- 

 tion of various appendages. In many Penaeidea the lamellar 

 mandibular palps, the antennal scales, and the antennular peduncles 

 co-operate to this end ; in the Brachyura and some Anomura the 

 flattened third maxillipeds carry the exhalent channels at least as 

 far as the anterior margin of the buccal frame ; the flagella of the 

 antennules in some Hippidea and of the antennae in the Corystidae 

 form a long exhalent (or inhalent) siphon ; and in the Leucosiidae 

 among the Brachyura the inhalent as well as the exhalent channels 

 are carried forwards to the front of the head beneath the expanded 

 maxillipeds. Some special adaptations for aerial respiration will 

 be described below in connection with the circulatory system. 



Alimentary System. The stomodaeal " stomach " of the Decapods 

 is developed into a triturating and straining apparatus of great 

 complexity. The simplest form of the gastric armature appears to be 

 found in the Penaeid genus Cemtaspis (Fig. 166), recently studied by 

 Bonnier. Here the chitinous lining of the stomach, although pro- 

 vided with numerous internally projecting ridges armed with setae 

 and spinules, is nowhere indurated or calcified to form distinct 

 sclerites such as are found in other Decapods, and in so far it 



