/. CRUSTACEA: DEC APOD A. 



i31 



while the nnteiuiie proper have but, a single though usually much larger 

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

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

 I fig, 4;39, yd). 



Wlieu 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 ether appendages (fig. 404, /) are small, bira- 

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

 tlie first pair is reduced, but in the male except in Palinuridse this pair is 

 well developed, curiously modified, and serves as a oopulatory (intromit- 

 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 tliickness of the integument prevents diffuse respiration 

 and accounts for tlie numerous gills (fig. 437) wliieh are attached 



pdba 



pdh.n 



Fig 4.37,— rrills of Astacus exposed hy cutting away the branr-hiostegite. jjrt/j, ;,»., 

 podo- and pleurohranchia of the corresponding segments; r, rostrum; i, stallted 

 eyes; 'J, S, antennae; A-'i', niandil)les and maxilUe; 7-v, maxiUipeds; lu, lU, bases of 

 thoracic feet; 15, first pleopod. 



to the bases of tlie appendages (maxillipeds and w'alking feet) or 

 to the sides of the hody near tliem. (In the Thalassinidie — forms 

 near the Astacidfe — tlie 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 {brancldostegite) over 

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

 the second maxillre — the scajtliognatldte — jolays in this branchial 

 :!hamber and jmmps the water over the gills, the water flowing oitt 

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

 fact readily exjilained 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 trojiical land crabs which live almost ex- 

 clusively on land there is a true aeri;il respiration, the lining of 

 the gill chamber becoming modified into a kind of lung traversed 



