/. CRUSTACEA: DEC APOD A. 431 



while the antennae proper have but a single though usually much larger 

 flagellum. On the basal joint of the antennulae is the auditory organ 

 (p. 412), while the green gland opens on the basal joint of the antennae 

 ^flg. 439, gd). 



When the abdomen is not rudimentary (as in the crabs) the appendages 

 of the sixth abdominal segment together with the telson form a strong 

 caudal fin (fig. 439); the other appendages (fig. 404, 7) are small, bira- 

 mous organs to which, in the female, the eggs are attached. In the female 

 the first pair is reduced, but in the male except in Palinuridae this pair is 

 well developed, curiously modified, and serves as a copulatory (introm it- 

 tent) organ. The condition of these appendages as well as the openings of 

 the genital ducts on the base of the third walking foot in the female, 

 the fifth in the male serve at once to distinguish the sexes. Frequently 

 also the males have the larger pincers. 



The thickness of the integument prevents diffuse respiration 

 and accounts for the numerous gills (fig. 437) which are attached 



pdb.Q pdfc.13 



FIG 437. Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb. 

 podo- and pleurobranchia of the corresponding segments; r, rostrum; 1, stalked 

 eyes; , 3, antennae ; A-e, mandibles and maxillae; 7-9, maxillipeds; 10, #, bases of 

 thoracic feet; 15, first pleopod. 



to the bases of the appendages (maxillipeds and walking feet) or 

 to the sides of the body near them. (In the Thalassinidse forms 

 near the Astacidae the gills are on the abdominal appendages). 

 These gills are not visible externally, for the carapace extends 

 down on the sides of the body as a fold (branchiostegite) over 

 them, thus enclosing them in a branchial chamber. A process of 

 the second maxillae the scapliognathite plays in this branchial 

 chamber and pumps the water over the gills, the water flowing out 

 near the mouth. All decapods can live some time out of water, a 

 fact readily explained when we remember that they retain some 

 water in the gill chamber, which keeps the gills in a moist con- 

 dition. In some of the tropical land crabs which live almost ex- 

 clusively on land there is a true aerial respiration, the lining of 

 the gill chamber becoming modified into a kind of lung traversed 



