APPEiq"DAGES OF AETHEOPODA. 



243 



the Decapoda provided witli brancliial cavities tliere ai-e special 

 churning organs (flagella) on either side (Fig. 125, f), which reach 

 over the whole of the gills in the form of thin flat processes^ and are 

 attached to the base of a maxil- 

 liped, by which they are kept 

 constantly moving (Brachyura). 



The lamella3 of the integn- 

 mentj which in many Entomo- 

 straca carry shells, must be re- 

 garded as having a respiratory 

 significance. This relation to 

 respiration is intelligible when 

 we note that a considerable 

 amount of blood passes through 

 these lamella? of the mantle, and 

 that the thinness of the walls of 

 these organs presents a condition 

 weU adapted for the exchange of 

 the gases ; and that, further, the 

 movements of the appendages 

 effect a considerable exchange of 

 water within the mantle-chamber. 

 When the pallial lamellte become 

 more extended (Limnadiaceee) 

 they become of greater import- 

 ance for respiration, and this 

 importance must increase in pro- 

 portion to the reduction in num- 

 ber of the appendages, which 

 lose their respiratory significance 

 as less blood passes through them 

 (Ostracoda, Daphnida). 



In these cases, however, the 

 mantle does not become specially organised into a branchial organ, 

 as it does in the Cirripedia. In the Balanidte folded lamella?, Avhich 

 have been regarded as gills, appear on the inner surface of the 

 mantle- cavity, between its side-wall and base. 



Fig. 125. Branchiaa of a Brachyurons 

 Decapod. The dorsal integument has 

 been removed from the greater part of 

 the cephalothorax. In the middle is seen 

 the coelom with the intestine arising from 

 the masticatory stomach v ; the branchial 

 cavities are placed at the sides and laid 

 open ; on the right are the branchiae ar- 

 ranged in six lamellar rows ; on the left 

 four of them have been cut away, as well 

 as the flagellum /, so as to display the 

 churning apparatus / ' /", below the gills. 

 Eyes, d Antenna;, ar A single gill 

 cut short at re. 



Appendages of the Tracheata. 



§ 188. 



The appendages of the Tracheata are distinguished fi'om those of 

 the Crustacea by the absence of the terminal bifurcation, so that 

 they are composed of a single series of joints. In Peripatus these 

 joints are but feebly differentiated. The terminal portion only 

 which carries a claw has any large amount of independence. 



K 2 



