DECAPODA 367 



water which enters this opening is prevented from taking a short cut 

 to the exhalant passage by a large expansion of the base of the mastigo- 

 branch of the first maxiUiped, which directs it under the gills. The 

 current from the openings over the legs also passes under the 

 gills. All the water then passes upwards through the gills into the 

 epibranchial space above them and so to the exhalant passage. Thus 

 the gills are thoroughly bathed. 



Owing to the width of the body the sterna are more easily dis- 

 tinguished than in the crayfish. Those of the maxillulary to second 



pb.mp.3 



pb.mp.2 ^^^ \ / p/5,T 



ar.ch. 



Fig. 273. A dorsal view of the organs in the left branchial chamber of Car- 

 ciniis maenas. From Borradaile. ah. abdomen; ar.ch. arthrobranchs of the 

 cheliped; ar.7np.2, arthrobranch of the second maxilliped; ar.inp.^, arthro- 

 branchs of the third maxilliped; ep.i, epipodite of the first maxilliped; i.r. 

 innerlayer of the branchiostegal fold, reflected •,pb. ynp. 2, ph. nip. 2, podohr^inchs 

 of the first and second maxillipeds; ^c./. pericardial lobe, a thin fold of the body 

 wall, of undetermined function; ^/6. i, plh.2, first and second pleurobranchs ; 

 p.hy. posterior opening of the hypobranchial chamber; scl. sclerite which 

 keeps open the entrance to the exhalant passage; scp. scaphognathite. 



maxillipedal somites are fused into a triangular mass. In front of the 

 mouth the plate known as the epistome represents the mandibular and 

 antennal sterna. From this a ridge extends to the median rostral 

 tooth, separating two sockets in which stand the antennules. A down- 

 ward process from the front, abutting on the basal joint of the antenna, 

 separates each of these sockets from the orbit of its side. The two- 

 jointed eyestalk arises close to the median line and passes through a 

 gap between the frontal process and the antennal base to enlarge 

 within the orbit. 



