240 



COMPARATIVE ANATOMY. 



joint; which projects over the 



Fig. 123. Appendages of Astacus 

 fluviatilis, seen from the ventral 

 surface, at Anterior; at' Posterior 

 antenna, md 1 Mandibular piece, mp" 

 Third maxilliped, which covers over 

 the other gnathites. P 1 — P 5 Ambu- 

 latory-feet. %i- — p s Swimming-feet 

 of the abdomen, p 6 Telson. a Anus. 

 o Orifice of tho oviduct in the basal- 

 joint of the third ambulatory foot. 



ultimate one; this arrangement is 

 generally most marked on the first 

 pair of feet, where it serves as an 

 organ of offence. The ambulatory 

 feet, as well as the niaxillipedes, 

 have branchial tufts appended to 

 them. 



A number of feet on the abdo- 

 men are converted into delicate 

 swimming-feet ; the first of them 

 functions in the male as a copu- 

 latory organ ; in the female it is 

 reduced in size, while the other four 

 (p- — p 5 ) carry the ova. The last 

 pair of appendages are the most 

 modified, for they (p 6 ) form together 

 with the last segment of the body 

 a powerful caudal fin, of which they 

 are the sides. 



The other divisions of the 

 Malacostraca present more or less 

 marked deviations from this ar- 

 rangement in the number of their 

 gnathites or of the appendages, 

 which are converted into locomotor 

 organs and adapted to these func- 

 tions. Thus, for example, in the 

 wood-louse four appendages are 

 converted into gnathites, the next 

 eight are ambulatory, and the last 

 four form respiratory plates. 



The connection between respi- 

 ration and locomotion, as implied 

 by the conversion of appendages 

 into branchial lamellas, or in the 

 differentiation of gills of various 

 form ou the appendages, is a 

 most important phamoinenon (see 

 Branchia?) . 



Branchite. 



§ 180. 



The tendency to longitudinal division which affects the appen- 

 dages of the Crustacea adapts these structures to the respiratory as 

 well as to the locomotor function, by spreading out their joints. 

 When the integument at certain points gets thinner it gives rise to 



