DECAPODA. 



517 



the middle line, and project externally to the second antennae 



(Fig. 316). They are lodged in hollows of the cephalo- thorax, the 



orbits, but the structures bounding the latter vary in the several 



groups. The anterior, external, and posterior walls of the orbit 



may, as we have seen, be regarded as formed by the pleural 



fold, the excavation of the margin having become very much 



deeper than it is in the Macrura. 



In most Brachyura the basal 



segments of the second antenna 



are situated at the postero-inter- 



nal angle of the orbit, and thus 



furnish a part of its boundary ; 



the enlarged basal segments of 



the first antenna completing the 



inner walls (Fig. 316). 



In the Catometopa the down- 

 turned front of the carapace 

 bounds the orbits internally, and 

 a blunt spine-like process from the 

 posterior orbital margin, project- 

 ing forwards and inwards, more 

 or less completely excludes the 

 second antenna from participa- 

 tion in the orbital boundary. 



In the Ocypodidae the orbits 

 are greatly extended laterally to 

 form " orbital grooves," lodging 

 the long and highly mobile 

 eyes. 



Finally in the Oxystomata 

 there is no posterior orbital wall, the pleural fold being absent 

 in this region. 



Branchiae. In Anaspides each thoracic leg bears, as has been 

 shown, two epipodial gills, having a simple leaf -like form (Fig. 

 287). In the larva of Penaeus, the branchiae make their appear- 

 ance as simple lobes, of which there are at first three, attached 

 to the bases of each of the central thoracic limbs. To these a 

 fourth is soon added, Fig. 317, lying near the proximal one. Of 

 the original three, the middle branchia is attached to the region 

 which subsequently becomes the membrane of the articulation 



?1G. 317. The rudiments of the 

 branchiae of Penaeus (after Claus). 

 la-7a podobranchiae ; lc-7c and 8c' 

 pleuro branchiae, inmost segments two 

 to each appendage ; the arthrobranchiae 

 are seen between the podobranchiae 

 and pleurobranchiae ; p l -p$ trunk legs ; 

 pm l -pm s maxillipeds. 



